Modify test_flatfield_step_interface step to not interfere with other tests

[braingram]

The test_flatfield_step_interface was unskipped in #7615 right before the frequent but unpredictable failures for FlatField.

This test modifies FlatFieldStep.reference_file_types (and does not undo the modification). Subsequent tests (in the same process) that use FlatField may fail (and not consider f/s/dflats) which would explain the errors seen in regression tests following the merge of the above changes.

This minimal regression test run (2 tests in one process) will hopefully help to explain the error:
https://plwishmaster.stsci.edu:8081/blue/organizations/jenkins/RT%2FJWST-Developers-Pull-Requests/detail/JWST-Developers-Pull-Requests/799/tests
The order of tests was controlled so that test_flatfield_step_interface was called prior to test_nis_wfss_spec2 (which fails with the expected error).

This PR modifies the test to save, modify then restore FlatFieldStep.reference_file_types so that it doesn't interfere with other tests.

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[braingram]

Regression tests running: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/802/

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (d1a60ee) 76.60% compared to head (c021c8e) 76.60%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7752   +/-   ##
=======================================
  Coverage   76.60%   76.60%           
=======================================
  Files         456      456           
  Lines       36950    36950           
=======================================
+ Hits        28304    28305    +1     
+ Misses       8646     8645    -1     

Flag	Coverage Δ		*Carryforward flag
nightly	77.43% <ø> (ø)		Carriedforward from d1a60ee

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see 1 file with indirect coverage changes

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[nden]

😱
Good catch!

[braingram]

Thanks for the review!

It looks like I'm not authorized to merge this. The regression tests are almost done (stuck on the last 98%) but should finish in about 30 minutes (and so far no errors have been hit).

I did run these tests with --durations=10 to see the names of the slowest tests. Based on previous runs I expect it's one of these (based off their previous runtimes) which have not so far shown the d/s/fflat errors.

1hr9min [stable-deps] test_spec3_multi[jw01249005001_03101_00001_nrs1_o005_crf.fits] – jwst.regtest.test_nirspec_ifu_spec3
1hr [stable-deps] test_residual_fringe_cal – jwst.regtest.test_miri_residual_fringe 

[hbushouse]

It works! YAY!!!