Fix an error caused by a bug in Python < 2.7.9

There seems to be a bug in Python versions prior
to 2.7.9 where running exec could produce the following error:

SyntaxError: unqualified exec is not allowed in function
'_ParsePythonTestTemplates' it contains a nested function
with free variables (testcfg.py, line 71)

https://bugs.python.org/issue21591

It's causing an issue on all Ubuntu 14 and RHEL 7 machines.

The proposed change is an equivalent syntax which doesn't
produce an error:
https://docs.python.org/2/reference/simple_stmts.html#the-exec-statement


Change-Id: I159cc1be58ff375f313ae5c4fb814763704b880e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1893647Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#64736}

test/preparser/testcfg.py

@@ -68,7 +68,7 @@ class TestSuite(testsuite.TestSuite):
       return MkTest
     with open(pathname) as f:
       text = f.read()
-    exec(text, {"Test": Test, "Template": Template})
+    exec text in {"Test": Test, "Template": Template}
 
   def ListTests(self):
     result = []