Fail tests if still running after 5 minutes

When diagnosing build-time tests that get stuck, such as the one in
!12, it's helpful for the test to abort after some reasonable timeout
(leaving a core dump and any previous log output), rather than relying on
some external process to kill the entire build after a longer arbitrary
timeout like an hour (which would also terminate whatever test harnesses,
logging, etc. are in use, limiting the available diagnostics).

The implementation of egg_tests_set_fatal_timeout() is heavily based
on code that I contributed to dbus. The choice of a 5 minute timeout is
completely arbitrary (dbus mostly uses 1 minute): please adjust up or
down as appropriate.

If a particular test case is known to be particularly slow, you can call
egg_tests_set_fatal_timeout() in setup() and/or teardown() to reset the
timer, effectively providing a per-test-case timeout. I haven't done
this in any of the tests.

When running under valgrind or qemu, or on a particularly slow CPU,
the environment variable TEST_FATAL_TIMEOUT_MULTIPLIER can be set to
an integer to multiply all of these timeouts. For example,
TEST_FATAL_TIMEOUT_MULTIPLIER=10 has worked well when running dbus tests
under valgrind, and 20 works well for qemu-system-risc64.

Signed-off-by: Simon McVittie <smcv@debian.org>