Document test strategy

[ChristianKuehnel]

Document the test strategy of the LLVM project. Link to other documents where we already have something.

Testing stages I know of:

• local test before submitting a patch
• pre-merge testing on Phabricator
• post-merge testing on buildbots, ...
• release testing (CI and manual)
• downstream testing (TensorFlow, Rust, within companies, linux distros, ...)

For each stage document who does what when.

This would also be the fondation to improve testing. We can then precisely argue which part of the testing process we want to change.