tests: Use temporary directories in unit tests

[kratman]

Description

The unit tests leave a large number of files behind in the git repo. This change will make the tests use temporary directories when creating files.

I used the tempfile module in python. There are a few other ways to do this, and I believe the preferred method is to use pytest. However, since pytest does not appear to be used, I went with tempfile as the easy solution.

Type of change

Minor fix in the tests.

• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• No style issues: $ pre-commit run (or $ nox -s pre-commit) (see CONTRIBUTING.md for how to set this up to run automatically when committing locally, in just two lines of code)
• All tests pass: $ python run-tests.py --all (or $ nox -s tests)
• The documentation builds: $ python run-tests.py --doctest (or $ nox -s doctests)

You can run integration tests, unit tests, and doctests together at once, using $ python run-tests.py --quick (or $ nox -s quick).

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[kratman]

This will have conflicts with #3373, so it should be reviewed/merged after that one is complete.

[kratman]

Conflicts resolved, this should be ready.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Comparison is base (c5aa3ce) 99.56% compared to head (708f721) 99.58%.
Report is 47 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3377      +/-   ##
===========================================
+ Coverage    99.56%   99.58%   +0.01%     
===========================================
  Files          253      254       +1     
  Lines        19561    19817     +256     
===========================================
+ Hits         19476    19734     +258     
+ Misses          85       83       -2     

Files	Coverage Δ
pybamm/__init__.py	100.00% <100.00%> (ø)
pybamm/solvers/base_solver.py	100.00% <100.00%> (ø)
pybamm/solvers/idaklu_solver.py	100.00% <100.00%> (+0.90%)	⬆️
pybamm/solvers/processed_variable.py	100.00% <100.00%> (ø)
pybamm/solvers/processed_variable_computed.py	100.00% <100.00%> (ø)
pybamm/solvers/solution.py	100.00% <100.00%> (ø)

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

[kratman]

Lychee error is unrelated to this change

[kratman]

@Saransh-cpp Can this be removed so that these files do not keep getting created when I test?

[Saransh-cpp]

Sorry for the delay, @kratman, I'll review this before the end of this week. The extra files should not be a problem given that they should be ignored by git? Is there a specific redundant file that is not being ignored in .gitignore?

[kratman]

None are missed by gitignore, just they exist until I run a cleanup command

[Saransh-cpp]

Thanks for working on this, looks amazing, @kratman!