Unit test for TranslationBranchApprover not very easy to understand

The unit test TestTranslationBranchApprover is very hard to read for thos not familiar with it. One main cause for that are the helper funtions _upload_template and _upload_second_template that do much more than their names suggest, depending on certain conditions.

These methods should be split up so that the steps to set up each individual test method become clearer. For example, the reader of test_replace_existing_approved should understand the following setup steps by reading the code:

1. A potemplate entry is created in the database with domain A and path YB
2. An upload is simulated by putting a file with path C into the import queue.
3. The approver is created with path D as its parameter.