stap_run.exp: accept 'WARNING: Child process exited due to signal'

stap_run.exp: accept 'WARNING: Child process exited due to signal'

This was causing numerous spurious FAIL outcomes on RHEL[89].
The extra "WARNING: Child process exited due to signal" is not an error,
it's merely stap communicating to the user that the child
process was interrupted, which is exactly what we do.

Not sure why it's printed on some systems and not others.
XXX There may be a deeper issue in that the tiny do-nothing binary
probed by the testcase (e.g. testsuite/systemtap.base/at_register.c)
was supposed to stop on its own and didn't.
Adding a delay to the first kill -INT $mypid makes the problem
more obvious.