[BEAM-12985] Call pytest directly rather than run_pytest.sh to avoid running pyarrow tests twice

[TheNeuralBit]

Additionally, this PR adds a check to run_pytest.sh to fail early if called with -m, so we don't make this mistake in the future:

❯ ./scripts/run_pytest.sh pyarrow "-m uses_pyarrow"
./scripts/run_pytest.sh cannot be called with -m as it interferes with 'no_xdist' logic, see BEAM-12985.

ValidatesRunner compliance status (on master branch)

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Examples testing status on various runners

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Post-Commit SDK/Transform Integration Tests Status (on master branch)

Go	Java	Python

Pre-Commit Tests Status (on master branch)

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See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

GitHub Actions Tests Status (on master branch)

See CI.md for more information about GitHub Actions CI.

[codecov]

Codecov Report

Merging #15693 (3bfeebc) into master (cac6b50) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #15693      +/-   ##
==========================================
+ Coverage   83.49%   83.50%   +0.01%     
==========================================
  Files         445      445              
  Lines       61194    61194              
==========================================
+ Hits        51093    51103      +10     
+ Misses      10101    10091      -10     

Impacted Files	Coverage Δ
sdks/python/apache_beam/internal/metrics/metric.py	90.00% <0.00%> (-1.00%)	⬇️
...hon/apache_beam/runners/worker/bundle_processor.py	93.64% <0.00%> (+0.12%)	⬆️
...runners/interactive/display/pcoll_visualization.py	86.36% <0.00%> (+0.50%)	⬆️
...pache_beam/runners/interactive/interactive_beam.py	76.58% <0.00%> (+0.97%)	⬆️
...che_beam/runners/interactive/interactive_runner.py	92.59% <0.00%> (+1.85%)	⬆️
...hon/apache_beam/runners/direct/test_stream_impl.py	96.26% <0.00%> (+2.23%)	⬆️
.../python/apache_beam/testing/test_stream_service.py	93.02% <0.00%> (+4.65%)	⬆️

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[udim]

Nit: consider adding {posargs} to the end of this line. It might not be super useful in this case because I'm guessing this runs quickly, but it allows to pass additional arguments to pytest (as in: tox -e py38-pyarrow-5 -- -k my_test).

[TheNeuralBit]

Done.