explicitly set the profile name to avoid random test errors

[escattone]

mozilla/sumo#1479

This PR fixes another random test-case failure. The UsernamesTests.test_query_old test case initiates a case-insensitive search for users whose username or profile.name starts with the letter a. In the setup for the test, a single user is created with an explicitly given username but a randomly generated profile.name. The test expects no users to be found, but since the profile.name is randomly generated, sometimes it's given a name that starts with the letter a, and the test fails. So the fix is simply to explicitly set the profile.name in the test setup.

I also took the opportunity to improve the quality of the UsernamesTests.test_query_current test case.