This commit forbids the changing of thread pool types for any thread
pool. The motivation here is that these are expert settings with
little practical advantage.
Closes#14294, relates #2509, relates #2858, relates #5152
The test jar was previously built in maven by copying class files. With
gradle we now have a proper test framework artifact. This change moves
the classes used by the test framework into the test-framework module.
See #13930