In PhpUnit tests using a @dataProvider fooProvider within a test class marked as @group $Database, the code in the data provider function fooProvider will access the production DB throughout MW's usage of the global DB object.
E.g. when using User::createNew( 'John' ) in the data provider will create a new user "John" which will be available in the production wiki but not during the actual test.

The test DB should already be in place during data provider execution.