s4: torture: Fix race condition in test_smb2_kernel_oplocks8.

s4: torture: Fix race condition in test_smb2_kernel_oplocks8.

The child process gets the kernel lease and then notifies
the parent process to continue by writing a byte up a pipe.
It then sets the alarm and calls pause() to wait for the
parent process to contact the smbd and get it to trigger
the break request using an open call.

It is possible for the parent to run and trigger the break
request after the child has written to the pipe, but *before*
the child calls pause(). We then miss the signal notifying
the child to break the lease.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13121

Signed-off-by: Jeremy Allison <jra@samba.org>
Reviewed-by: Ralph Böhme <slow@samba.org>