Squashing unit tests failures occurring on Mac OS X.

The changes simply emulate the setup and teardown as done in the other
testing module for PageTemplates, testHTMLTests. I am not 100% sure if the fix
is the correct thing to do but it makes the tests run on Mac OS X and
does not affect them on Linux.

The main reason behind the failures is most likely some other test that runs
before which fiddles with the security policies but never cleans up
afterwards. I did some poking but could not find anything.

lib/python/Products/PageTemplates/tests/testDTMLTests.py

@@ -17,6 +17,7 @@ from Products.PageTemplates.tests import util
 from Products.PageTemplates.PageTemplate import PageTemplate
 from Acquisition import Implicit
 from AccessControl import SecurityManager
+from AccessControl.SecurityManagement import noSecurityManager
 
 class AqPageTemplate(Implicit, PageTemplate):
    pass
@@ -48,9 +49,11 @@ class DTMLTests(unittest.TestCase):
       self.t=(AqPageTemplate())
       self.policy = UnitTestSecurityPolicy()
       self.oldPolicy = SecurityManager.setSecurityPolicy( self.policy )
+      noSecurityManager()  # Use the new policy.
 
    def tearDown(self):
       SecurityManager.setSecurityPolicy( self.oldPolicy )
+      noSecurityManager()  # Reset to old policy.
 
    def check1(self):
       """DTML test 1: if, in, and var: