posix/tst-spawn: Fix racy tests in spawned processes.

posix/tst-spawn: Fix racy tests in spawned processes.

From time to time I get fails in tst-spawn like:
tst-spawn.c:111: numeric comparison failure
   left: 0 (0x0); from: xlseek (fd2, 0, SEEK_CUR)
  right: 28 (0x1c); from: strlen (fd2string)
error: 1 test failures
tst-spawn.c:252: numeric comparison failure
   left: 1 (0x1); from: WEXITSTATUS (status)
  right: 0 (0x0); from: 0
error: 1 test failures

It turned out, that a child process is testing it's open file descriptors
with e.g. a sequence of testing the current position, setting the position
to zero and reading a specific amount of bytes.

Unfortunately starting with commit 2a69f853c03034c2e383e0f9c35b5402ce8b5473
the test is spawning a second child process which is sharing some of the
file descriptors.  If the test sequence as mentioned above is running in parallel
it leads to test failures.

As the second call of posix_spawn shall test a NULL pid argument,
this patch is just moving the waitpid of the first child
before the posix_spawn of the second child.

ChangeLog:

	* posix/tst-spawn do_test(): Move waitpid before posix_spawn.