Add ReadMe tip for speeding up unit tests

[JeremyTCD]

OS?

Windows 10

Version

v1.0.0-beta.20-1

Repro steps

1. Run ng test with chrome in focus.

2. Run ng test with chrome minimized.

3. Test run time can differ by more than an order of magnitude.

Other details that might be useful

I was running ~200 tests (30 of them were component tests). Execution time was either ~0.7s or ~20s and the test runner would eventually crash. Turns out the slow and unstable tests were a result of chrome being out of focus. Chrome windows that are out of focus get throttled.

Looking through the rebuild speed thread, #1980, I noticed some complaints about ng test test speed. Specifically this comment. I'd like to suggest including a tip in ReadMe to keep chrome in focus. Since this is quite a hard to diagnose issue I think it will be very helpful.

[filipesilva]

The problem with ng test in that thread was mostly fixed by #2840, because at the time code coverage was always being added.

A comment like this would be nice, even though there isn't anything else we can do. Would you be interested in adding it?

[antonybudianto7]

no blueprint update after https://github.com/angular/angular/pull/16250/files ? @filipesilva

[filipesilva]

Newer karma-chrome-launcher versions fix this automatically.

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