t/Makefile: fix result handling with TEST_OUTPUT_DIRECTORY
When TEST_OUTPUT_DIRECTORY is set, the test results will be generated in
"$TEST_OUTPUT_DIRECTORY/test-results", which may not be the same as
"test-results" in t/Makefile. This causes the aggregate-results target
to fail as it finds no count files.
Fix this by introducing TEST_RESULTS_DIRECTORY and using it wherever
test-results is referenced.
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>