Pass exit code through reporters to karma

[ryanvall]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes / features) - project doesn't appear to have any, tests in tests dir aren't testing lib behavior
• Docs have been added / updated (for bug fixes / features) - doesn't seem necessary here unless something needs to be in the releases log (doesn't appear to have been updated in a while)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Bug fix

• What is the current behavior? (You can also link to an open issue here)

Possibly related to #35, but karma-parallel doesn't appropriately exit with code 1 if coverage thresholds aren't met when this is used in conjunction with karma-coverage. In this issue/subsequent PR support was added in karma/karma-coverage to pass the exit code through in the onExit callback:

karma-runner/karma-coverage#418
karma-runner/karma#3541
karma-runner/karma-coverage@c93f061#diff-691854e392b02b6faf6147eef5d32e3bb9f187025dc025742167d4bdce7443feR286

The existing implementation of onExit in karma-parallel doesn't pass along the returned values from onExit in the individual reporters.

• What is the new behavior (if this is a feature change)?

The exit code is now passed through from the original reporters.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Possibly? Users might now have commands/CI builds fail because of coverage, but that would be correct/no longer a false positive. I don't know enough about karma/the reporter API to know if onExit is only ever supposed to be passed an exit code, so if it can be passed other data/arguments this behavior wouldn't be entirely correct.

• Other information:

I'm not 100% sure if this is the right solution - this wouldn't be an issue for anyone not utilizing a coverage reporter/coverage thresholds. Open to feedback on how to improve this.

[dongyuwei]

I can confirm latest karma-parallel still has the bug, and this pull-request do fixed the bug.
Please merge and publish a new version, thanks.

Additional info:

"karma": "^6.4.1",

"karma-chrome-launcher": "~3.1.1",

"karma-coverage": "^2.2.0",

"karma-jasmine": "^5.1.0",

"karma-jasmine-html-reporter": "^1.7.0",

"karma-parallel": "^0.3.1",
"karma-viewport": "^1.0.6",