Bug 1176642 - Defer import of glob; r=glandium

This was the only import of glob from all mach_commands.py files. Kill
it.

With this commit, there are no modules imported by a single
mach_commands.py outside of testing/web-platform/mach_commands.py.

python/mach_commands.py

@@ -5,7 +5,6 @@
 from __future__ import absolute_import, print_function, unicode_literals
 
 import argparse
-import glob
 import logging
 import mozpack.path as mozpath
 import os
@@ -53,6 +52,7 @@ class MachCommands(MachCommandBase):
         help='Tests to run. Each test can be a single file or a directory.')
     def python_test(self, tests, verbose=False, stop=False):
         self._activate_virtualenv()
+        import glob
 
         # Python's unittest, and in particular discover, has problems with
         # clashing namespaces when importing multiple test modules. What follows