refactor(tests): Update bootstrap.js to better support generator chunks (#7171)

Refactor bootstrap.js and bootstrap_helper.js to be able to deal
with generator chunks.  In particular for each chunk, specify:

- The goog.module ID to goog.require() in uncompressed mode.
- The script filename to load in compressed mode.
- Where the chunk's UMD wrapper will save the export object when
  loaded as a script.
- What global variable the chunk's export object should be saved in
  (if desired).
- Any individual named exports to destructure to global variables.

This allows the bootstrap scripts to be slightly simpler while
also being more flexible.

tests/generators/index.html

@@ -5,7 +5,7 @@
 <title>Blockly Generator Tests</title>
 <script>
   var BLOCKLY_BOOTSTRAP_OPTIONS = {
-    additionalScripts: [
+    scripts: [
       'build/msg/en.js',
       'tests/generators/unittest_javascript.js',
       'tests/generators/unittest_python.js',