Fix step-over-{trips-on-watchpoint|lands-on-breakpoint}.exp race

Fix step-over-{trips-on-watchpoint|lands-on-breakpoint}.exp race

On a target that is both always in non-stop mode and can do displaced
stepping (such as native x86_64 GNU/Linux, with "maint set
target-non-stop on"), the step-over-trips-on-watchpoint.exp test
sometimes fails like this:

   (gdb) PASS: gdb.threads/step-over-trips-on-watchpoint.exp: no thread-specific bp: step: thread 1
   set scheduler-locking off
   (gdb) PASS: gdb.threads/step-over-trips-on-watchpoint.exp: no thread-specific bp: step: set scheduler-locking off
   step
  -[Switching to Thread 0x7ffff7fc0700 (LWP 11782)]
  -Hardware watchpoint 4: watch_me
  -
  -Old value = 0
  -New value = 1
  -child_function (arg=0x0) at /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.threads/step-over-trips-on-watchpoint.c:39
  -39           other = 1; /* set thread-specific breakpoint here */
  -(gdb) PASS: gdb.threads/step-over-trips-on-watchpoint.exp: no thread-specific bp: step: step
  +wait_threads () at /home/pedro/gdb/mygit/src/gdb/testsuite/gdb.threads/step-over-trips-on-watchpoint.c:49
  +49       return 1; /* in wait_threads */
  +(gdb) FAIL: gdb.threads/step-over-trips-on-watchpoint.exp: no thread-specific bp: step: step

Note "scheduler-locking" was set off.  The problem is that on such
targets, the step-over of thread 2 and the "step" of thread 1 can be
set to run simultaneously (since with displaced stepping the
breakpoint isn't ever removed from the target), and sometimes, the
"step" of thread 1 finishes first, so it'd take another resume to see
the watchpoint trigger.  Fix this by replacing the wait_threads
function with a one-line infinite loop that doesn't call any function,
so that the "step" of thread 1 never finishes.

gdb/testsuite/ChangeLog:
2015-08-07  Pedro Alves  <palves@redhat.com>

	* gdb.threads/step-over-lands-on-breakpoint.c (wait_threads):
	Delete function.
	(main): Add alarm.  Run an infinite loop instead of calling
	wait_threads.
	* gdb.threads/step-over-lands-on-breakpoint.exp (do_test): Change
	comment.
	* gdb.threads/step-over-trips-on-watchpoint.c (wait_threads):
	Delete function.
	(main): Add alarm.  Run an infinite loop instead of calling
	wait_threads.
	* gdb.threads/step-over-trips-on-watchpoint.exp (do_test): Change
	comment.