tests: ipc: speed the suite up with avoiding expendable sleep(3)s

Using i7-6820HQ CPU yields these results:

Before: ~2:54
After: ~2:26
Speedup: ~16%

The main optimization lies in how run_function_in_new_process helper is
constructed, since now, there's an actual synchronization between the
parent and its child (that needs to be prioritized here, which is
furthermore help with making the parent immediately give up it's
processor possession) after the fork, so that a subsequent sleep is
completely omitted -- at worst (unlikely), additional sleep round(s)
will need to be undertaken as already arranged for (and now, just
400 ms is waited rather than excessive 1 second).

Another slight optimization is likewise in omission of sleep where
the control gets returned to once the waited for process has been
suceesfully examined post-mortem, without worries it's previous
life is still resounding.

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>