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

[cpcallen]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide
• I ran npm run format and npm run lint

The details

Resolves

There are some issues with how tests/bootstrap.js loads modules in compiled and uncompiled mode:

• It does really relate goog.module IDs loaded in uncompressed mode vs. the compiled chunk file names loaded in compressed mode.
• It relied on certain quirks of how the generator chunks worked—specifically, that you can goog.require('Blockly.JavaScript.all') to load the whole chunk via it's official entry point, but then do var {javascriptGenerator} = goog.module.get('Blockly.JavaScript') to get the javascriptGenerator object, since it is exported from both Blockly.JavaScript and Blockly.JavaScript.all. This prevents certain desirable refactorings of the generator modules.

Proposed Changes

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.

Test Coverage

• Passes npm test.
• Main playground loads, generators work.
• Advanced playground loads but generators are broken there (but they were already: Generators in the advanced playground do not work #6597).

No special manual testing changes required: any additional problems will become abundantly clear!