fix: Fix mjs file support

[chrisbreiding]

• Closes Can't reexport the named export 'BREAK' from non EcmaScript module (only default export is available) when including aws-amplify - lack of mjs file support #8361

User facing changelog

• Fixed an issue where .mjs files would error

Additional Details

The true fix for this is in @cypress/webpack-batteries-included-preprocessor. This bumps its version and adds a test for .mjs files.

Also did some minor refactoring of the plugins e2e spec.

PR Tasks

• Have tests been added/updated?
• Has the original issue been tagged with a release in ZenHub?

[cypress-bot]

Thanks for taking the time to open a PR!

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.
• Become familiar with the Code Review Checklist for guidelines on coding standards and what needs to be done before a PR can be merged.

[cypress]

---

Test summary

8137 • 0 • 108 • 3

---

Run details

Project	cypress
Status	Passed
Commit	ec76ab2
Started	Aug 25, 2020 1:27 PM
Ended	Aug 25, 2020 1:34 PM
Duration	06:32 💡
OS	Linux Debian - 10.2
Browser	Multiple

View run in Cypress Dashboard ➡️

---

This comment has been generated by cypress-bot as a result of this project's GitHub integration settings. You can manage this integration in this project's settings in the Cypress Dashboard

[jennifer-shehane]

This issue has some links in the stack trace to .mjs file, wonder if it's related, but they didn't really give a repro #8373

I manually verified that this fixes the original issue. Tests look good. 👍

[mwmcode]

I know this is not the right place to ask, but is there any branch where upgrading to webpack 5 is being tested?

Webpack has progressed to ^5.0.0-rc.3 and webpack-preprocessor seems incompatible with it (some vague error about Automatic publicPath is not supported in this browser)

I tried pulling and upgrading webpack-preprocessor locally, but to no avail :(