[tests] Infer test name from first test source file

Florian Schmaus requested to merge flow/emper:test-fixes into master

This also fixes a bug in source = test_dict['source'][0] so the source(s) used for the test was always only the first source file. I did/does not matter, as we do not have tests that span multiple source files (and I am not sure if we ever will have).

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