Fix npm test if os.cpus().length == 1

[jablko]

Currently it complains

[09:10:20] > node built/local/run.js

TypeScript/src/testRunner/runner.ts:232
        beforeEach(() => {
        ^
ReferenceError: beforeEach is not defined

This change synchronizes scripts/build/tests.jsGulpfile.js with the logic in src/testRunner/runner.ts.

[sandersn]

Is the problem that runtests-parallel doesn't work on systems with only 1 CPU? I think runtests should work in this case -- is there some automated context where switching between the two is difficult?

(Having to ask these questions now is one reason why we like to have bugs for every PR, even housekeeping ones.)

[jablko]

Sorry, npm test doesn't work on systems with only one CPU. This change effectively makes it use runtests if workers == 1, instead of erroring.

[sandersn]

Ah, I didn't see that it was npm test. That's a common entry point for first-time contributors, so it's important for it to work.