Screenshot test support

See gr-file-list_screenshot_test.ts for an example of how to create a
screenshot test. When a test fails it creates a gitignore'd file of the
diff heatmap. These tests are excluded from the normal test command
while we figure out how to normalize screenshots across operating
systems for the CI, besides this they could easily be included in our
standard test files.

Run tests excluding screenshot tests:
> yarn test

Run tests including screenshot tests:
> yarn test:screenshot

Run test and update goldens:
> yarn test:screenshot-update "**/gr-file-list_screenshot_test.ts"

Release-Notes: skip
Change-Id: If9e755f051e2af3906a27b6306ff0811a78a3c68
diff --git a/polygerrit-ui/yarn.lock b/polygerrit-ui/yarn.lock
index 68b6fff..ca6943d 100644
--- a/polygerrit-ui/yarn.lock
+++ b/polygerrit-ui/yarn.lock
@@ -1526,6 +1526,13 @@
   resolved "https://registry.yarnpkg.com/@types/minimatch/-/minimatch-3.0.5.tgz#1001cc5e6a3704b83c236027e77f2f58ea010f40"
   integrity sha512-Klz949h02Gz2uZCMGwDUSDS1YBlTdDDgbWHi+81l29tQALUtvz4rAYi5uoVhE5Lagoq6DeqAUlbrHvW/mXDgdQ==
 
+"@types/mkdirp@^1.0.1":
+  version "1.0.2"
+  resolved "https://registry.yarnpkg.com/@types/mkdirp/-/mkdirp-1.0.2.tgz#8d0bad7aa793abe551860be1f7ae7f3198c16666"
+  integrity sha512-o0K1tSO0Dx5X6xlU5F1D6625FawhC3dU3iqr25lluNv/+/QIVH8RLNEiVokgIZo+mz+87w/3Mkg/VvQS+J51fQ==
+  dependencies:
+    "@types/node" "*"
+
 "@types/mocha@^8.2.0":
   version "8.2.3"
   resolved "https://registry.yarnpkg.com/@types/mocha/-/mocha-8.2.3.tgz#bbeb55fbc73f28ea6de601fbfa4613f58d785323"
@@ -1551,6 +1558,20 @@
   resolved "https://registry.yarnpkg.com/@types/path-is-inside/-/path-is-inside-1.0.0.tgz#02d6ff38975d684bdec96204494baf9f29f0e17f"
   integrity sha512-hfnXRGugz+McgX2jxyy5qz9sB21LRzlGn24zlwN2KEgoPtEvjzNRrLtUkOOebPDPZl3Rq7ywKxYvylVcEZDnEw==
 
+"@types/pixelmatch@^5.2.2":
+  version "5.2.4"
+  resolved "https://registry.yarnpkg.com/@types/pixelmatch/-/pixelmatch-5.2.4.tgz#ca145cc5ede1388c71c68edf2d1f5190e5ddd0f6"
+  integrity sha512-HDaSHIAv9kwpMN7zlmwfTv6gax0PiporJOipcrGsVNF3Ba+kryOZc0Pio5pn6NhisgWr7TaajlPEKTbTAypIBQ==
+  dependencies:
+    "@types/node" "*"
+
+"@types/pngjs@^6.0.0":
+  version "6.0.1"
+  resolved "https://registry.yarnpkg.com/@types/pngjs/-/pngjs-6.0.1.tgz#c711ec3fbbf077fed274ecccaf85dd4673130072"
+  integrity sha512-J39njbdW1U/6YyVXvC9+1iflZghP8jgRf2ndYghdJb5xL49LYDB+1EuAxfbuJ2IBbWIL3AjHPQhgaTxT3YaYeg==
+  dependencies:
+    "@types/node" "*"
+
 "@types/qs@*":
   version "6.9.7"
   resolved "https://registry.yarnpkg.com/@types/qs/-/qs-6.9.7.tgz#63bb7d067db107cc1e457c303bc25d511febf6cb"
@@ -1780,6 +1801,14 @@
     "@web/test-runner-core" "^0.10.27"
     mkdirp "^1.0.4"
 
+"@web/test-runner-commands@^0.6.4":
+  version "0.6.5"
+  resolved "https://registry.yarnpkg.com/@web/test-runner-commands/-/test-runner-commands-0.6.5.tgz#69a2a06b52fd9d329f9cf1e172cd8fb1d5ffc521"
+  integrity sha512-W+wLg10jEAJY9N6tNWqG1daKmAzxGmTbO/H9fFfcgOgdxdn+hHiR4r2/x1iylKbFLujHUQlnjNQeu2d6eDPFqg==
+  dependencies:
+    "@web/test-runner-core" "^0.10.27"
+    mkdirp "^1.0.4"
+
 "@web/test-runner-core@^0.10.20":
   version "0.10.22"
   resolved "https://registry.yarnpkg.com/@web/test-runner-core/-/test-runner-core-0.10.22.tgz#34bb67d12a79b01dc79c816f3d76f3419ef50eaf"
@@ -1862,6 +1891,20 @@
     "@types/mocha" "^8.2.0"
     "@web/test-runner-core" "^0.10.20"
 
+"@web/test-runner-visual-regression@^0.6.6":
+  version "0.6.6"
+  resolved "https://registry.yarnpkg.com/@web/test-runner-visual-regression/-/test-runner-visual-regression-0.6.6.tgz#4a4dc734f360cba66a005e07b4a1c0a9ef956444"
+  integrity sha512-010J3zE6z2v7eLLey/l5cYa9/WhPsgzZb3Z6K5nn4Mn5W5LGPs/f+XG3N6+Tx8Q1/RvDqLdFvRs/T6c4ul4dgQ==
+  dependencies:
+    "@types/mkdirp" "^1.0.1"
+    "@types/pixelmatch" "^5.2.2"
+    "@types/pngjs" "^6.0.0"
+    "@web/test-runner-commands" "^0.6.4"
+    "@web/test-runner-core" "^0.10.20"
+    mkdirp "^1.0.4"
+    pixelmatch "^5.2.1"
+    pngjs "^6.0.0"
+
 "@web/test-runner@^0.14.0":
   version "0.14.0"
   resolved "https://registry.yarnpkg.com/@web/test-runner/-/test-runner-0.14.0.tgz#fc7b206f3fdc5a1d774cfc8f60159a574d30b185"
@@ -4569,6 +4612,13 @@
   resolved "https://registry.yarnpkg.com/pify/-/pify-3.0.0.tgz#e5a4acd2c101fdf3d9a4d07f0dbc4db49dd28176"
   integrity sha512-C3FsVNH1udSEX48gGX1xfvwTWfsYWj5U+8/uK15BGzIGrKoUpghX8hWZwa/OFnakBiiVNmBvemTJR5mcy7iPcg==
 
+pixelmatch@^5.2.1:
+  version "5.3.0"
+  resolved "https://registry.yarnpkg.com/pixelmatch/-/pixelmatch-5.3.0.tgz#5e5321a7abedfb7962d60dbf345deda87cb9560a"
+  integrity sha512-o8mkY4E/+LNUf6LzX96ht6k6CEDi65k9G2rjMtBe9Oo+VPKSvl+0GKHuH/AlG+GA5LPG/i5hrekkxUc3s2HU+Q==
+  dependencies:
+    pngjs "^6.0.0"
+
 pkg-dir@4.2.0:
   version "4.2.0"
   resolved "https://registry.yarnpkg.com/pkg-dir/-/pkg-dir-4.2.0.tgz#f099133df7ede422e81d1d8448270eeb3e4261f3"
@@ -4576,6 +4626,11 @@
   dependencies:
     find-up "^4.0.0"
 
+pngjs@^6.0.0:
+  version "6.0.0"
+  resolved "https://registry.yarnpkg.com/pngjs/-/pngjs-6.0.0.tgz#ca9e5d2aa48db0228a52c419c3308e87720da821"
+  integrity sha512-TRzzuFRRmEoSW/p1KVAmiOgPco2Irlah+bGFCeNfJXxxYGwSw7YwAOAcd7X28K/m5bjBWKsC29KyoMfHbypayg==
+
 polyfills-loader@^1.7.4:
   version "1.7.6"
   resolved "https://registry.yarnpkg.com/polyfills-loader/-/polyfills-loader-1.7.6.tgz#5cff98bfc9689cf10e44bdd32f498cfeb4374c51"