Patch obvious missing tests

[RasmusSkytte]

Intent

This PR adds simple tests for the parts of our code base that is not covered by tests.
This is not intended to imply that this code is well-tested, but it will introduce a minimum level of testing for this code

Approach

Tests added / modified:

• DiseasyRegions$plot() is tested to not produce errors
• DiseasyActivity$plot() is tested to not produce errors
• DiseasyImmunity$describe() is tested to not produce errors
• The test for hash_environment() wraps the error-inputs in lists to correctly mimic the how errors would be passed to the function. The current implementation of the tests was wrong and some code would not be reached in the tests.
• Test with malformed input added for combineasy()
• All providers of optimisers tested in DiseasyImmunity$approximate_compartmental()
• All tests where the comments say we use exponential_waning now actually use exponential_waning.

In addition:

• .covrignore added to not flag missing coverage in our linters (since these are only for developers)

Known issues

N/A

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.83%. Comparing base (6c1fcdd) to head (f1a411e).
⚠️ Report is 12 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #291       +/-   ##
===========================================
+ Coverage   64.17%   78.83%   +14.65%     
===========================================
  Files          28       27        -1     
  Lines        9597     4483     -5114     
===========================================
- Hits         6159     3534     -2625     
+ Misses       3438      949     -2489     

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