Allow test_loader_name to override test class module

[cdent]

When running gabbi by default the pretty name of the test
(displayed in test results) has a 'gabbi.suitemaker' prefix. This
can be disorienting when reviewing result sets or trying to track
those results to where in a code tree the gabbi tests are loaded.

This change allows the module of the test classes that are
dynamically created to be override with the test_loader_name
that is passed in to to the build_tests method. By setting this
to name the test is appropriately situated in its context. The
gist of this change can be seen on the diff to test-limit.sh.

Note that this has also been tested in OpenStack Nova, which
was the source of this need. There tests which were showing up with
the name:

gabbi.suitemaker.test_placement_api_inventory_get_that_inventory.\
  test_request

have become:

nova.tests.functional.api.openstack.placement.\
  test_placement_api.inventory_get_that_inventory.test_request

By default nothing has changed. This change only impact situations where
test_loader_name is used.

/cc @FND @jasonamyers @sdague

[jasonamyers]

I think this is great. I've explained this to others before.