feat: make mocha fail if it encounters 0 tests

[maribethb]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide

The details

Resolves

Proposed Changes

If mocha encounters 0 tests, it fails

Behavior Before Change

If mocha encounters 0 tests, it reports 0 failures and CI passes

Behavior After Change

If mocha encounters 0 tests, it says 1 tests failed now and fails CI.

Reason for Changes

Catches scenarios where somehow mocha configuration is broken, no tests are run, and we are blissfully unaware since CI reports no problems.

Test Coverage

Tested via commenting out all of the goog.require lines on index.html so none of the tests are imported. The browser shows 0 tests running, but the command line reports the 1 failure as expected.

Documentation

n/a

Additional Information

n/a

[cpcallen]

Can this go in now? I think it just needs to have the develop branch merged in, since #5982 has been merged.