Merge #17384: test: Create new test library

fa4c6fa9b1 doc: Add documentation for new test/lib (MarcoFalke)
faec28252c 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 fa4c6fa9b1 -- diff looks correct and Travis is happy
  jonatack:
    ACK fa4c6fa9b1 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 fa4c6fa9b1. 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

src/test/util_tests.cpp

@@ -6,7 +6,7 @@
 
 #include <clientversion.h>
 #include <sync.h>
-#include <test/setup_common.h>
+#include <test/util/setup_common.h>
 #include <test/util.h>
 #include <util/moneystr.h>
 #include <util/strencodings.h>