Replace Karma with Web Test Runner on CI

yarn test:single is no longer auto-globbed so you would do
yarn test:single "**/gr-button_test.ts" or similar.

- run_tests.sh is called by CI, swapped to WTR bazel target.
- In package.json added ":karma" suffix to old tests, replaced with new
  ones.
- Test fix: The root HTML element will not have .lightTheme class. We
  could add it back, but WTR is doing good isolation here causing tests
  to fail.
- Test fix: JS tests don't use .js extension when importing relative
  files.
- Test fix: Fix gr-search-bar_test to actually use ConfigModel
  subscription instead of default base URL. Karma seemed to be relying
  on some window object thing before so test was failing under WTR.

Release-Notes: skip
Change-Id: I2575807825648066d58442003b0b7d032112707a
diff --git a/polygerrit-ui/app/run_test.sh b/polygerrit-ui/app/run_test.sh
index 0ceca3c..916fc82 100755
--- a/polygerrit-ui/app/run_test.sh
+++ b/polygerrit-ui/app/run_test.sh
@@ -13,4 +13,4 @@
       "$@" \
       --test_verbose_timeout_warnings \
       --test_output=all \
-      //polygerrit-ui:karma_test
+      //polygerrit-ui:web-test-runner