Further unit test fixes

[netomi]

This PR fixes a few further unit test problems that I encountered:

• gather method for redis backend works fine, so enabled the test
• ensure files generated during test are created in tmp directory
• ensure that all method calls during test execution happen within a context manager to properly close the cache (otherwise a sqlite connection hangs around in some cases)

With all these fixes, running all tests in my IDE successfully completes without any hanging thread.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (8ed3214) 97.21% compared to head (8f787d0) 97.21%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #186   +/-   ##
=======================================
  Coverage   97.21%   97.21%           
=======================================
  Files          10       10           
  Lines         934      934           
  Branches      171      171           
=======================================
  Hits          908      908           
  Misses         15       15           
  Partials       11       11           

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