Fix karma tests memory issues

[barklund]

Task Description

Karma tests often have memory issues and halt execution - even before actually starting running the tests.

This seems inconsistent but increasingly common, as tests grow more and more complex and comprehensive.

Possible fixes include:

• Increasing memory limit
• Fixing underlying memory leak (if any)

See this thread for discussion.

[dvoytenko]

I think I can confirm that it happens in the source map preprocessing. Here's the full debug log for the failure:

>> WebPack
✔ Stories Editor
  Compiled successfully in 133.37ms

30 06 2020 09:10:48.198:DEBUG [web-server]: Instantiating middleware
ℹ ｢wdm｣: Compiled successfully.
ℹ ｢wdm｣: Compiling...

✔ Stories Editor
  Compiled successfully in 58.80s

ℹ ｢wdm｣: Compiled with warnings.

>>> Fails between these two logs

30 06 2020 09:12:03.278:DEBUG [preprocessor.sourcemap]: base64-encoded source map for /Users/dvoytenko/DNI/web-stories-wp/assets/src/edit-story/components/canvas/karma/backgroundCopyPaste.karma.js

This most likely comes from this Karma configuration:

preprocessors: {
  'assets/src/edit-story/**/karma/**/*.js': ['webpack', 'sourcemap'],
},

Not clear what are all issues related to sourcemap here.

[dvoytenko]

Here's something similar and suggested solution is here. This promises to speed up our test compilation time on WebPack quite a bit.

[barklund]

The above configuration generates a webpack bundle for each test.

That sounds horrible! Yes, let's go for this alternative approach and pack them all in single bundle for CI.

[dvoytenko]

PTAL: #2887