test-lib-functions: detect test_when_finished in subshell

test-lib-functions: detect test_when_finished in subshell

test_when_finished does nothing in a subshell because the change to
test_cleanup does not affect the parent.

There is no POSIX way to detect that we are in a subshell ($$ and $PPID
are specified to remain unchanged), but we can detect it on Bash and
fall back to ignoring the bug on other shells.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>