Update to @atom/mocha-test-runner and mocha@6.0.2

[smashwilson]

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Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.
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Description of the Change

Change our test runner dependency from @smashwilson/atom-mocha-test-runner to @atom/mocha-test-runner. Update Mocha to match the version from the new runner.

Screenshot/Gif

Alternate Designs

N/A

Benefits

Most immediately, it'll address some lagging dependencies and transitive dependencies, and fix #1965.

Possible Drawbacks

There's the possibility of breaking changes. This is a major-version bump for Mocha.

Applicable Issues

Fixes #1965.

Metrics

N/A

Tests

• Verify that the graphical test runner works.
• Verify that the headless test runner works.
• Verify that our istanbul integration still works (by ensuring that the CodeCov results look correct).

Documentation

N/A

Release Notes

N/A

User Experience Research (Optional)

N/A

[codecov]

Codecov Report

Merging #1991 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1991      +/-   ##
==========================================
- Coverage   92.12%   92.12%   -0.01%     
==========================================
  Files         188      188              
  Lines       10809    10806       -3     
  Branches     1582     1581       -1     
==========================================
- Hits         9958     9955       -3     
  Misses        851      851

Impacted Files	Coverage Δ
lib/git-shell-out-strategy.js	100% <0%> (ø)	⬆️

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[smashwilson]

The build is green, aside from some wonkiness with the macOS beta build agent and the status check reporting.