Track and report test coverage

[nylen]

Use Jest code coverage added in #1382 (with some tweaks/improvements in this PR to ignore built files, Storybook stories, etc.) and coveralls.io and node-coveralls for reporting coverage during Travis CI builds.

[coveralls]

Changes Unknown when pulling 6cda42908d786e2cc5a4ce7969651c9cb837b2d6 on add/coverage-badge into ** on master**.

[coveralls]

Changes Unknown when pulling f765a03c7dbeca0109760d8128ebfe3752579a40 on add/coverage-badge into ** on master**.

[coveralls]

Changes Unknown when pulling 9453c971dcdcceb24c58328d812a76ead9de6e38 on add/coverage-badge into ** on master**.

[ntwb]

I see .vscode was already here and this is just moving its location, though should it really be here in the first place?

Via #1382 (comment) from a couple of days ago:

IDE specific entries in .gitignore should not be in the repo, users should manage these locally in ~/.gitignore_global

See also https://core.trac.wordpress.org/ticket/34345

[nylen]

👍 Originally added in #294.

[ntwb]

Coveralls comments and notifications can get noisy, I think turning off comments and changing the failure threshold to 5% would be something we should do:

Via https://coveralls.io/github/WordPress/gutenberg/settings
• Uncheck the "LEAVE COMMENTS?" checkbox
• Change the "COVERAGE DECREASE THRESHOLD FOR FAILURE" value to 5%

[ntwb]

LGTM 👍

[nylen]

Coveralls comments and notifications can get noisy, I think turning off comments and changing the failure threshold to 5% would be something we should do

Maybe so. However, coverage is at 18%. Eighteen percent. This shows that we are not developing with testing in mind.

So I'd like to leave this enabled for a while.

[coveralls]

Changes Unknown when pulling 3a1cc23 on add/coverage-badge into ** on master**.

[coveralls]

Changes Unknown when pulling 3a1cc23 on add/coverage-badge into ** on master**.

[aduth]

With these changes, local testing and Travis testing don't produce the same result. You can see that running npm test locally fails on the master branch with an error "Your test suite must contain at least one test" (because it's trying to run e.g. test/setup-globals.js as a test, fix pending). I see that npm run test-unit:coverage-ci runs the test, but does not fail with the same error.

[aduth]

Hmm, actually, npm run test-unit:coverage-ci does fail with the same error for me. Not sure why yet...