[lldb][NFC] Specify guidelines for API tests

This patch specifies a few guidelines that our API tests should follow.

The motivations for this are twofold:

1. API tests have unexpected pitfalls that especially new contributors run into
when writing tests. To prevent the frustration of letting people figure those
pitfalls out by trial-and-error, let's just document them briefly in one place.

2. It prevents some arguing about what is the right way to write tests. I really
like to have fast and reliable API test suite, but I also don't want to be the
bogeyman that has to insist in every review that the test should be rewritten to
not launch a process for no good reason. It's much easier to just point to a
policy document.

I omitted some guidelines that I think could be controversial (e.g., the whole
"should assert message describe failure or success").

Reviewed By: shafik

Differential Revision: https://reviews.llvm.org/D101153

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