Windows support for build_test

[stdbug]

Create a portable empty_test rule that always succeeds.
Use that as the test portion fo build_test

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[stdbug]

@googlebot I signed it!

[googlebot]

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[stdbug]

@googlebot I consent.

[aiuto]

@sdtwigg You might want to look at this too, WRT to the build test refactoring you did recently.

[stdbug]

@juliexxia Does this need anything else from my side?

[juliexxia]

Apologies for the delay! Adding @meteorcloudy for windows domain knowledge. I think he's OOO until thursday though.

[meteorcloudy]

Nice, thanks for the contribution!

[stdbug]

@googlebot I consent.

[aiuto]

Everyone seems to like this. Can we sort out the CLA part? This sometimes happens if different users push to the same PR. For example, from a corporate account and a personal account.

[FaBrand]

@codeschlosser You could try to close this PR and create a new one with a squashed branch. Since you probably have consented already this could work. I would also be happy to have this in skylib 👍

[stdbug]

@FaBrand, done #302