test.ganeti.process_unittest: Fix race condition

There was a race condition on heavily loaded testsystem causing randomly
to fail the timeout unittests as the signal handler is not yet setup but
the timeout has already hit.

Therefore we introduce a workaround to wait until a program reached a
certain point (for example after signal handling setup) before we
actually go for the real run. The wait of course has a timeout as well,
but it's pretty high. If we hit the 20 seconds we have really big issues
anyway.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>