bpo-33615: Skip test__xxsubinterpreters (pythonGH-7503)

The test does crash on multiple CIs causing many troubles. For
example, the test prevents to get results of the two Refleak 3.x
buildbots.

Author: vstinner

Lib/test/test__xxsubinterpreters.py

@@ -12,6 +12,8 @@
 from test import support
 from test.support import script_helper
 
+raise unittest.SkipTest("FIXME: bpo-33615: test crash on some CIs")
+
 interpreters = support.import_module('_xxsubinterpreters')