Allow option to run template test on individual files

Also adds some basic documentation on using the bazel test target.

Sample usage:
bazel test //polygerrit-ui/app:template_test --test_output errors \
--test_arg=gr-list-view

Change-Id: I013f4c4e8411b6f34afb31e9baae3bb30dea5ad4

polygerrit-ui/app/template_test_srcs/template_test.js

@@ -42,9 +42,21 @@ fs.readdir('./polygerrit-ui/temp/behaviors/', (err, data) => {
     });
   }
 
-  const mappings = JSON.parse(fs.readFileSync(
+  let mappings = JSON.parse(fs.readFileSync(
       `./polygerrit-ui/temp/map.json`, 'utf-8'));
 
+  // If a particular file was passed by the user, don't test everything.
+  const file = process.argv[2];
+  if (file) {
+    const mappingSpecificFile = {};
+    for (key of Object.keys(mappings)) {
+      if (key.includes(file)) {
+        mappingSpecificFile[key] = mappings[key];
+      }
+    }
+    mappings = mappingSpecificFile;
+  }
+
   externs.push({
     path: 'custom-externs.js',
     src: '/** @externs */' +