Enable process isolation for unit tests.

test/runner/lib/executor.py

@@ -624,6 +624,7 @@ def command_units(args):
 
         cmd = [
             'pytest',
+            '--boxed',
             '-r', 'a',
             '--color',
             'yes' if args.color else 'no',

test/runner/requirements/units.txt

@@ -6,6 +6,7 @@ passlib
 pycrypto
 pytest
 pytest-mock
+pytest-xdist
 python-memcached
 pyyaml
 redis