[lldb] [test] Fix variable overwrite in non-stop fork tests

Thanks to Pavel Labath for noticing the mistake in:
https://reviews.llvm.org/D128638#3618039

Sponsored by: The FreeBSD Foundation

lldb/test/API/tools/lldb-server/TestGdbRemoteForkNonStop.py

@@ -92,8 +92,7 @@ def test_kill_all_nonstop(self):
             "send packet: $OK#00",
         ], True)
         self.expect_gdbremote_sequence()
-        self.assertEqual(set([ret["pid1"], ret["pid2"]]),
-                         set([parent_pid, child_pid]))
+        self.assertEqual(set([pid1, pid2]), set([parent_pid, child_pid]))
 
     @add_test_categories(["fork"])
     def test_vkill_both_nonstop(self):