add pytest no-crds option to disable tests that require crds

[braingram]

This PR adds a no-crds pytest option to disable tests that require downloading ~500MB of files from CRDS.

Checklist

• added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (ec2cd1b) 65.22% compared to head (12591ce) 65.22%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #118   +/-   ##
=======================================
  Coverage   65.22%   65.22%           
=======================================
  Files         101      101           
  Lines        5450     5450           
=======================================
  Hits         3555     3555           
  Misses       1895     1895           

Impacted Files	Coverage Δ
src/stdatamodels/jwst/datamodels/dqflags.py	100.00% <ø> (ø)

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[braingram]

The option is off by default so that when pytest is run with no options, tests against crds will be run (and data will be downloaded when needed). The crds tests are only skipped if a user explicitly passes the --no-crds option.

This is useful because the test suite without the crds tests (on my machine) take ~48 seconds. With downloading data the crds tests alone take 218 seconds (490 seconds if using the vpn shared cache).

The crds tests contribute 0% to the test coverage.

Including this no-crds option will allow for much faster local testing.

[braingram]

Thanks!