Update the nosetests test regex for legacy tests

The default regex, '(?:^|[\b_\./-])[Tt]est', will match methods that are
not test methods. For example, I created a new method
'skip_test_overrides' and it was mistakenly run as test. By convention
all of our tests start with 'test_' so this is a safe change.

Change-Id: I4f5917de53681c950c72479f16f88e76f49f6bfc

tox.ini

@@ -39,13 +39,13 @@ deps = -r{toxinidir}/test-requirements.txt
 commands =
   # Run each legacy test separately, to avoid SQL model redefinitions
   find keystone -type f -name "*.pyc" -delete
-  nosetests -v \
+  nosetests -v -m '^[Tt]est' \
       keystone/tests/unit/backend/legacy_drivers/assignment/V8/sql.py
-  nosetests -v \
+  nosetests -v -m '^[Tt]est' \
       keystone/tests/unit/backend/legacy_drivers/role/V8/sql.py
-  nosetests -v \
+  nosetests -v -m '^[Tt]est' \
       keystone/tests/unit/backend/legacy_drivers/federation/V8/api_v3.py
-  nosetests -v \
+  nosetests -v -m '^[Tt]est' \
       keystone/tests/unit/backend/legacy_drivers/resource/V8/sql.py
 
 [testenv:pep8]