DM-50999: Allow PipelineStepTester to accept a URI

[timj]

Checklist

• ran Jenkins
• ran and inspected package-docs build
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

❌ Patch coverage is 0% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.90%. Comparing base (c69b346) to head (6eb3295).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
python/lsst/pipe/base/tests/pipelineStepTester.py	0.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #545      +/-   ##
==========================================
+ Coverage   88.87%   88.90%   +0.02%     
==========================================
  Files         154      154              
  Lines       21603    21606       +3     
  Branches     2562     2562              
==========================================
+ Hits        19200    19208       +8     
+ Misses       1787     1784       -3     
+ Partials      616      614       -2     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.