doc: Add documentation for new test/lib

Author: MarcoFalke

src/test/README.md

@@ -7,8 +7,8 @@ configure some other framework (we want as few impediments to creating
 unit tests as possible).
 
 The build system is set up to compile an executable called `test_bitcoin`
-that runs all of the unit tests. The main source file is called
-`setup_common.cpp`.
+that runs all of the unit tests. The main source file for the test library is found in
+`util/setup_common.cpp`.
 
 ### Compiling/running unit tests
 

src/test/util/README.md

@@ -0,0 +1,11 @@
+# Test library
+
+This contains files for the test library, which is used by the test binaries (unit tests, benchmarks, fuzzers, gui
+tests).
+
+Generally, the files in this folder should be well-separated modules. New code should be added to existing modules or
+(when in doubt) a new module should be created.
+
+The utilities in here are compiled into a library, which does not hold any state. However, the main file `setup_common`
+defines the common test setup for all test binaries. The test binaries will handle the global state when they
+instantiate the `BasicTestingSetup` (or one of its derived classes).