Merge #17384: test: Create new test library

fa4c6fa9b1139791f45f1495d662c1c7cd2f7ed6 doc: Add documentation for new test/lib (MarcoFalke)
faec28252cf4f8e754c689a7a44fd421f631db50 scripted-diff: test: Move setup_common to test library (MarcoFalke)

Pull request description:

  Sorry for clickbait, this is only a move-only scripted-diff commit and one documentation commit.

  Longer term, someone who knows something about build systems can make this an actual library. Motivation for this is that each module gets compiled for each target that includes it. For example, setup_common is compiled 27 times (for the fuzz suite) and another 3 times for the other tests (bench, unit test, gui)

ACKs for top commit:
  practicalswift:
    ACK fa4c6fa9b1139791f45f1495d662c1c7cd2f7ed6 -- diff looks correct and Travis is happy
  jonatack:
    ACK fa4c6fa9b1139791f45f1495d662c1c7cd2f7ed6 with the reserve that the commit messages (and PR description) contain the motivation for this change. Built, ran tests, light code review.
  ryanofsky:
    Code review ACK fa4c6fa9b1139791f45f1495d662c1c7cd2f7ed6. I didn't realize `lib` was actually name of existing directory, not a new name. But in any case this looks good and nice to have one scripted diff instead of two.

Tree-SHA512: 2e176df90c60578276e4a6dc83ff57ff59d8e666ecf30c5ceacb8c326725da91baa4cac3dfa7a2e1605f58122a3e3e27e4938ff33e3a0ce7ea53afffebbf57a4

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