Move tests to aws-smithy-mocks

[simonmarty]

Issue #, if available:

Description of changes: Accidentally closed #112 on a force push.

Move unit tests to aws-smithy-mocks for easier test coverage.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

❌ Patch coverage is 97.92388% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.35%. Comparing base (0d67a90) to head (332b9e3).

Files with missing lines	Patch %	Lines
aws_secretsmanager_caching/src/lib.rs	97.92%	8 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #119      +/-   ##
==========================================
+ Coverage   91.72%   92.35%   +0.62%     
==========================================
  Files          14       14              
  Lines        2418     2747     +329     
  Branches     2418     2747     +329     
==========================================
+ Hits         2218     2537     +319     
- Misses        150      157       +7     
- Partials       50       53       +3     

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