tests: Don't leave a libguestfs tmpdir lying around after running test-launch-race.pl.

Calling _exit(2) in the child process has the side effect that
tmp/libguestfsXXXXXX is not cleaned up.  Clean it up by ensuring the
handle is properly closed before _exit.

tests/protocol/test-launch-race.pl

@@ -42,7 +42,9 @@ if ($pid == 0) {
   my $g = Sys::Guestfs->new ();
   $g->add_drive ("/dev/null");
   $g->launch ();
-  _exit (0); # So the tmpdir is not removed.
+  $g->close ();
+  # So $tmpdir is not removed by CLEANUP => 1 above.
+  _exit (0);
 }
 
 my $g = Sys::Guestfs->new ();