Add regression test for #666

Background

The issue #666 (closed) was a hot fix which needed to be released with 1.2.1, however it really should come with a regression test.

Task description

The regression test I have in mind should go into tests/frontend/fetch.py, and should be a parameterized test which tests:

• That a source with a ref already set, passes it's fetch routine without crashing

• This should be tested with, and without project.refs ref-storage mode

• This should be tested with, and without specifying a mirror in the associated project.conf

  • The mirror setting for project.conf should setup an alias set for the associated source under test
  • There should also be a parameterization of the test, which sets up a mirror in project.conf that sets a different alias, not associated with the source under test

• Write the test

Acceptance Criteria

The test passes.

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