Terminate individual tests after (10 * $TEST_TIMEOUT_FACTOR) minutes

Terminate individual tests after (10 * $TEST_TIMEOUT_FACTOR) minutes

While using the Automake parallel test harness, if a test hangs for
long enough for an external watchdog to kill the entire build process
(as happens in Debian sbuild after 150 minutes with no activity on
stdout/stderr), the logs will not be shown. If we make an individual
test time out sooner, logs are more likely to be shown.

We use SIGABRT so that the process(es) under test will dump core,
allowing the point at which ostree is blocking to be analyzed.
After 1 minute, if any have not died, we kill them again with SIGKILL.

To support slow platforms and slow debugging tools, if
TEST_TIMEOUT_FACTOR is set, multiply the 10 minute timeout by that.

Signed-off-by: Simon McVittie <smcv@debian.org>
Closes: #607
Approved by: cgwalters