update load_local_pkg exclude regex with regtest pattern

[braingram]

Fixes #8348

test_suffix.py tests that call jwst.stpipe.utilities.find_suffixes are skipped in unit test runs due to importorskip calls in the regtests (like):

jwst/jwst/regtest/test_stp_jw00697013.py

Line 30 in bf3b06f

pytest.importorskip('pysiaf')

pysiaf is not installed for CI unit test runs and the call to find_suffixes results in an attempt to import every .py file in the jwst directory structure that is not in a test folder (so regtest is included). This results in an attempt to import for example the above referenced file, the importorskip is called and the test (in this case one in test_suffix) is skipped.

This PR updates the regex to also exclude the regtest directory.

As a comparison here is the latest py310-xdist run log on main:
https://github.com/spacetelescope/jwst/actions/runs/8238566482/job/22529809746#step:10:248
showing:

==== 2785 passed, 554 skipped, 4 xfailed, 79 warnings in 854.19s (0:14:14) =====

With this PR the py310-xdist run log:
https://github.com/spacetelescope/jwst/actions/runs/8254717816/job/22579513473?pr=8350
shows 2 tests were unskipped:

==== 2787 passed, 552 skipped, 4 xfailed, 79 warnings in 905.68s (0:15:05) =====

The 2 tests that now run in the unit tests CI are:

• test_suffix_existence
• test_all_steps

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 run: https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/1313/
This shows 25 errors which all look unrelated and caused by:

• this PR not yet being rebased after: JP-3463: Add average_dark_current handling to dark_current step #8302
• crds update (changing some reference files)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.31%. Comparing base (4cc0ac1) to head (9abb746).
Report is 26 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8350      +/-   ##
==========================================
+ Coverage   75.15%   75.31%   +0.15%     
==========================================
  Files         470      474       +4     
  Lines       38604    38965     +361     
==========================================
+ Hits        29014    29346     +332     
- Misses       9590     9619      +29     

Flag	Coverage Δ		*Carryforward flag
nightly	77.33% <ø> (-0.07%)	⬇️	Carriedforward from 4cc0ac1

*This pull request uses carry forward flags. Click here to find out more.

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[hbushouse]

LGTM