Fix gdb.base/fork-running-state.exp race

Fix gdb.base/fork-running-state.exp race

On my multi-target branch I was occasionaly seeing a FAIL like this:

  (gdb) PASS: gdb.base/fork-running-state.exp: detach-on-fork=off: follow-fork=parent: non-stop: kill parent
  [Inferior 2 (process 32672) exited normally]
  kill inferior 2
  warning: Inferior ID 2 is not running.
  (gdb) FAIL: gdb.base/fork-running-state.exp: detach-on-fork=off: follow-fork=parent: non-stop: kill child (the program exited)
  ... other similar fails ...

Turns out to be a testcase bug/race.  A tweak like this increases the
changes of hitting the race substancially:

  --- a/gdb/testsuite/gdb.base/fork-running-state.c
  +++ b/gdb/testsuite/gdb.base/fork-running-state.c
  @@ -29,7 +29,7 @@ fork_child (void)
   {
     while (1)
       {
  -      sleep (1);
  +      usleep (100);

The testcase has two processes, parent and child fork.  The problem is
that the child exits itself if it notices the parent is gone, but the
testcase .exp does not expect that.

I first wrote a patch that handled the different combinations of
non-stop/detach-on-fork/follow-fork/schedule-multiple, making the .exp
file know when to expect the child to exit itself vs when to kill it
explicitly, but the result was that the code to kill the parent and
child was getting about as large as the test code that is the actual
point of the testcase, above the kills.

So I scratched that approach and came up with a simpler patch --
simply make the child not exit itself when the parent exits.

The .exp file is going to kill both parent and child explicitly, and,
main() already calls alarm() as a safeguard.  I don't think we lose
anything.

gdb/testsuite/ChangeLog:
2018-04-10  Pedro Alves  <palves@redhat.com>

	* gdb.base/fork-running-state.c (fork_child): Don't exit if parent
	exits.  Instead loop running forever.
	(fork_parent): Run forever too.