feat(Command test runner): Add command test runner

[nicojs]

Add the command test runner. This test runner simply runs command from the bash/cmd prompt. It defaults to npm test.

It will always report exactly one test result (called 'All tests'), as it cannot distinguish between the tests. It is implemented differently from other test runners, as we don't want to spawn this one in a separate process (it already uses require('child_process').exec to start a test run).

Fixes #768

[simondel]

What are your thoughts on the integration with stryker init and the use of a test framework in your config?

[nicojs]

For stryker init, I was thinking to put it as a last option:

• Use npm test (or other command)

How would we communicate that this option is much slower?

Configuring a test framework doesn't make sense anymore. The test runner does not support test selection. Good point, I will implement a fail fast for this.

[nicojs]

I've altered the first question to look like this:

This should be clear enough. If you select "command", the test framework question is skipped.

I've created #1049 for preset support. Let's not do that in this PR.

[nicojs]

I just totally forgot about the timeout functionality. Implemented now.

[DiegoRBaquero]

Finally got to review this! Looks great! Did you have any issues with zombie sub sub processes?

[simondel]

Do you have an open source project where you had these issues? Because then we could try it out :) we are pretty thorough with killing processes.

[DiegoRBaquero]

I don't quite remember which one it was exactly, but for sure it's in the top 20 most depended upon npm modules and I think it ran karma. I can't really remember which one it was. I should have documented it, sorry :(

[simondel]

No problem :)

[simondel]

This code is really clean. Nice job @nicojs! 💖