tests/run-tests: Disable thread/stress_heap.py when running on Travis.

It has reliability issues and sometimes fails on Travis (reason currently
unknown).

tests/run-tests

@@ -201,6 +201,7 @@ def run_tests(pyb, tests, args):
     if os.getenv('TRAVIS') == 'true':
         skip_tests.add('basics/memoryerror.py')
         skip_tests.add('thread/thread_lock4.py') # has reliability issues
+        skip_tests.add('thread/stress_heap.py') # has reliability issues
 
     if not has_complex:
         skip_tests.add('float/complex1.py')