use temporary directory for exports

[jhdark]

Proposed changes

With this change exports in the tests will be passed into a temporary directory rather than exporting locally. Reducing the number of files generated in the local space.

Types of changes

What types of changes does your code introduce to FESTIM?

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code refactoring
• Documentation Update (if none of the other choices apply)
• New tests

Checklist

• Black formatted
• Unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.08%. Comparing base (0573b74) to head (ab8dc46).
Report is 201 commits behind head on fenicsx.

Additional details and impacted files

@@           Coverage Diff            @@
##           fenicsx     #968   +/-   ##
========================================
  Coverage    96.08%   96.08%           
========================================
  Files           46       46           
  Lines         2608     2608           
========================================
  Hits          2506     2506           
  Misses         102      102           

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[RemDelaporteMathurin]

why not use pytest and tmpdir?