Fix test_negative_update_invalid

[mirekdlugosz]

Basically, the same thing as #386 and #387, but on another test.

This test creates a bunch of credentials, then creates a source and tries to update source to become invalid. There's no particular reason to use SSH key-backed credential specifically, we only need a valid credential.

$ pytest -v -ra 'camayoc/tests/qpc/api/v1/sources/test_network_sources.py::test_negative_update_invalid'
camayoc/tests/qpc/api/v1/sources/test_network_sources.py::test_negative_update_invalid[localhost] PASSED                                                               [ 33%]
camayoc/tests/qpc/api/v1/sources/test_network_sources.py::test_negative_update_invalid[127.0.0.1] PASSED                                                               [ 66%]
camayoc/tests/qpc/api/v1/sources/test_network_sources.py::test_negative_update_invalid[example.com] PASSED                                                             [100%]

============================================================================= 3 passed in 26.56s =============================================================================

[codecov[bot]]

Codecov Report

Merging #388 (8e283c8) into master (106b6f8) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #388   +/-   ##
=======================================
  Coverage   79.91%   79.91%           
=======================================
  Files           5        5           
  Lines         229      229           
=======================================
  Hits          183      183           
  Misses         46       46           

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