tests: remove useless 2nd subtest from DummyValgrindInvalidSupFile
tests: try to fix test failures due to stale files

These tests have the same binary directory as always. But until yesterday they
had a source directory in the source tree, now their source and binary
directory are the same and the CMakeLists.txt is written into them using
configure_file(). Wipe these directories before writing the configuration to
avoid errors because of stale files.
Merge topic 'MemChecker-improvements' into next

5a768f7 tests: try to fix test failures due to stale files
00a3038 tests: remove useless 2nd subtest from DummyValgrindInvalidSupFile