Mock out migrate.Command with --no-migrations

[blueyed]

It would still add some overhead (especially with regard to output
noise).

[codecov-io]

Codecov Report

Merging #729 into master will increase coverage by 1.13%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #729      +/-   ##
=========================================
+ Coverage   96.76%   97.9%   +1.13%     
=========================================
  Files          32      32              
  Lines        1856    1861       +5     
  Branches      147     147              
=========================================
+ Hits         1796    1822      +26     
+ Misses         44      26      -18     
+ Partials       16      13       -3

Impacted Files	Coverage Δ
tests/test_db_setup.py	100% <100%> (+3.77%)	⬆️
pytest_django/fixtures.py	96.37% <100%> (+0.07%)	⬆️
tests/test_environment.py	100% <0%> (+1.86%)	⬆️
pytest_django/plugin.py	94.02% <0%> (+2.59%)	⬆️
tests/conftest.py	100% <0%> (+5.17%)	⬆️
pytest_django/migrations.py	100% <0%> (+20%)	⬆️

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