Run PolyGerrit tests with Buck

Since Buck has no native support for web tests, wrap the tests in a
Python shim that calls wct. As in other Polymer cases, we need to
prepare a set of inputs and create a directory containing exactly what
wct expects to be there. Buck exposes resources to python_tests using
the pkg_resources API, which is cumbersome to use, and easier just to
ship around zip files as we do elsewhere.

Unlike other npm binaries we've encountered, the web-component-tester
module has numerous native dependencies, up to and including Selenium.
Rather than get in the game of distributing platform-specific
binaries, punt and require `wct` to be on the user's $PATH for now.

Tests are currently excluded in .buckconfig but can be run directly
with either:

  buck test //polygerrit-ui/app:polygerrit_tests
  buck test --include web

Change-Id: Ia314213925ac27ff271374a96ed539fb2acf0187
diff --git a/polygerrit-ui/README.md b/polygerrit-ui/README.md
index 00ff612..c2cd4bd 100644
--- a/polygerrit-ui/README.md
+++ b/polygerrit-ui/README.md
@@ -1,5 +1,18 @@
 # PolyGerrit
 
+## Installing [Node.js](https://nodejs.org/en/download/)
+
+```sh
+# Debian/Ubuntu
+sudo apt-get install nodejs-legacy
+
+# OS X with Homebrew
+brew install node
+```
+
+All other platforms: [download from
+nodejs.org](https://nodejs.org/en/download/).
+
 ## Local UI, Production Data
 
 To test the local UI against gerrit-review.googlesource.com:
@@ -31,7 +44,27 @@
 
 ## Running Tests
 
+One-time setup:
+
 ```sh
-npm install -g web-component-tester
-wct
+# Debian/Ubuntu
+sudo apt-get install npm
+
+# OS X with Homebrew
+brew install npm
+
+# All platforms (including those above)
+sudo npm install -g web-component-tester
+```
+
+Run all web tests:
+
+```sh
+buck test --include web
+```
+
+If you need to pass additional arguments to `wct`:
+
+```sh
+WCT_ARGS='-p --some-flag="foo bar"' buck test --no-results-cache --include web
 ```