Testsuite: Ensure interrupt-daemon-attach doesn't run forever

Testsuite: Ensure interrupt-daemon-attach doesn't run forever

Looking at the AArch64 buildbot, I noticed about two dozen old instances of
interrupt-daemon-attach taking up a full 100% cpu each.

If the test fails then the test binary relies on an alarm to ensure it dies
after 60 seconds.

As per the Linux man page for alarm:
  Alarms created by alarm() ... are not inherited by children created via fork.

Update the test to add an alarm in the child and also put a sleep in the
child loop so it does not constantly consume cpu.

Note I haven't managed to re-create why the test failed.  This fix will just
stop it hanging and consuming cpu when it does.

gdb/testsuite/ChangeLog:

	* gdb.base/interrupt-daemon-attach.c (main): Add alarm and sleep
	in child.