Infinite loop in sigstep.exp

[Michael Chastain]

This change seems to do the job:

  -  exp_continue
  +  exp_continue -continue_timer

"-continue_timer" means that exp_continue does not reset the timeout
counter for "expect".  So after the usual 120 seconds, gdb_test_multiple
busts out with a normal looking timeout FAIL.

I have to check that -continue_timer works with the vintage 1998
"expect" in sourceware.

ac> A [k]failing pattern for:
ac>
ac> 	"Breakpoint .*$gdb_prompt "
ac>
ac> would also help.

Yes, it would.  It would give a nice specific result, but just for this
one case.  If you want to submit a patch for that, go for it.

I'm gonna experiment some more with "-continue_timer" which ought to
break all cases of infinite loops.