t5562: do not reuse output files

t5562: do not reuse output files

Some expected failures of git-http-backend leaves running its children
(receive-pack or upload-pack) which still hold opened descriptors
to act.err and with some probability they live long enough to write
there their failure messages after next test has already truncated
the files. This causes occasional failures of the test script.

Avoid the issue by using separated output and error file for each test,
apprending the test number to their name.

Reported-by: Carlo Arenas <carenas@gmail.com>
Helped-by: Carlo Arenas <carenas@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Max Kirillov <max@max630.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>