Move testlib/selftests/expected_(.*)\.(.*) to testlib/selftests/$1/expected.$2

At present a listing of the qtbase/tests/auto/testlib/selftests/ directory is dominated by the 306 expected output files. It would be tidier to put each test's expected output in that test's directory instead.

Requires update to (at least) generate_expected_output.py, tst_selftests.cpp and CMakeLists.txt and careful testing.