test-lib: translate SIGTERM and SIGHUP to an exit

test-lib: translate SIGTERM and SIGHUP to an exit

Right now if a test script receives SIGTERM or SIGHUP (e.g., because a
test was hanging and the user 'kill'-ed it or simply closed the
terminal window the test was running in), the shell exits immediately.
This can be annoying if the test script did any global setup, like
starting apache or git-daemon, as it will not have an opportunity to
clean up after itself. A subsequent run of the test won't be able to
start its own daemon, and will either fail or skip the tests.

Instead, let's trap SIGTERM and SIGHUP as well to make sure we do a
clean shutdown, and just chain it to a normal exit (which will trigger
any cleanup).

This patch follows suit of da706545f7 (t: translate SIGINT to an exit,
2015-03-13), and even stole its commit message as well.

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