Convert ResourceRelatedField test to pytest styled tests

[sliverc]

Working in progress for #468

Description of the Change

Of test module test_relations for now only moved ResourceRelatedField related tests to keep the PR to a reasonable size.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

Merging #887 (46d0998) into master (5488b18) will decrease coverage by 0.06%.
The diff coverage is 98.80%.

@@            Coverage Diff             @@
##           master     #887      +/-   ##
==========================================
- Coverage   97.80%   97.73%   -0.07%     
==========================================
  Files          60       60              
  Lines        3325     3269      -56     
==========================================
- Hits         3252     3195      -57     
- Misses         73       74       +1     

Impacted Files	Coverage Δ
example/settings/dev.py	100.00% <ø> (ø)
tests/test_relations.py	98.85% <98.55%> (-1.15%)	⬇️
tests/conftest.py	100.00% <100.00%> (ø)

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[n2ygk]

Umm, this stuff is a little out of my league but, sure, let's do it.

[sliverc]

@n2ygk Thanks. Once you have an understanding of the basic pytest features such as fixtures this code becomes very simple. I recommend you to read a tutorial such as https://realpython.com/pytest-python-testing/ which summarizes the difference between unittest and pytest quite well.