DM-46025: Force a default AWS region for S3 tests

[timj]

This prevents problems when there is a local override in ~/.aws/config.

Note that we use "us-east-1" for all tests. "us-west-1" does not work. In theory MOTO_ALLOW_NONEXISTENT_REGION could be set to allow an arbitrary fake region to be used but that does not seem to work with our tests at present.

Checklist

• ran Jenkins
• added a release note for user-visible changes to doc/changes

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.01%. Comparing base (9adbd98) to head (bc211d0).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #93   +/-   ##
=======================================
  Coverage   87.01%   87.01%           
=======================================
  Files          27       27           
  Lines        4460     4460           
  Branches      905      905           
=======================================
  Hits         3881     3881           
  Misses        431      431           
  Partials      148      148           

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