Tests: Try to speed up CI tests execution

[gziolo]

From Tests are Extremely Slow on Docker and/or Continuous Integration (CI) server section from Jest Troubleshooting documentation:

Another alternative to expediting test execution time on Continuous Integration Servers such as Travis-CI is to set the max worker pool to ~4. Specifically on Travis-CI, this can reduce test execution time in half.

First try is to run all tests in the same thread using --runInBand (-i). On Travis with this change:

Second try is to run all tests with the max worker pool set to ~4. On Travis with this change:

Third try is to run all tests using unlimited number of workers as it is at the moment. Travis with this change:

[coveralls]

Coverage remained the same at 17.816% when pulling efdb78e on update/run-jest-in-band-ci into d157b5e on master.

[coveralls]

Coverage remained the same at 17.816% when pulling 17f2d9b on update/run-jest-in-band-ci into d157b5e on master.

[coveralls]

Coverage remained the same at 17.816% when pulling 9ffd99d on update/run-jest-in-band-ci into d157b5e on master.

[gziolo]

@nylen @ntwb It looks like we should limit number of workers used with Travis to run Jest. It looks like we should pick between 1-4 workers to improve performance. What would be your pick?

[nylen]

Let's go with 1 because we already have another task running during build, and Travis workers only have 2 cores. See #1808 for details.

[nylen]

Also, switching to npm 5 will get us another big speed boost (#1600).