t/Makefile: remove 'test-results' on 'make clean'

t/Makefile: remove 'test-results' on 'make clean'

The 't/test-results' directory and its contents are by-products of the
test process, so 'make clean' should remove them, but, alas, this has
been broken since fee65b194d (t/Makefile: don't remove test-results in
"clean-except-prove-cache", 2022-07-28).

The 'clean' target in 't/Makefile' was not directly responsible for
removing the 'test-results' directory, but relied on its dependency
'clean-except-prove-cache' to do that [1].  ee65b194d broke this,
because it only removed the 'rm -r test-results' command from the
'clean-except-prove-cache' target instead of moving it to the 'clean'
target, resulting in stray 't/test-results' directories.

Add that missing cleanup command to 't/Makefile', and to all
sub-Makefiles touched by that commit as well.

[1] 60f26f6348 (t/Makefile: retain cache t/.prove across prove runs,
                2012-05-02)

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>