Add a testing suite for diseasystores

[RasmusSkytte]

Intent

Fixes #90

Approach

The tests for DiseasystoreGoogleCovid19 was generalised and moved to the new function test_diseasystore().

This function takes any generator (R6 class specification) for a diseasystore and runs a standard set of tests on the diseasystore.

This allows us to quickly test new diseasystores as we create them.

Known issues

This PR also contains the code being merged in

• Add default value to diseasyoption #122

Once merged, this PR will be rebased and opened.

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

Attention: Patch coverage is 97.04641% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 95.00%. Comparing base (8e42ffd) to head (4775eb8).

Files	Patch %	Lines
R/test_diseasystore.R	97.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
+ Coverage   94.52%   95.00%   +0.47%     
==========================================
  Files          13       14       +1     
  Lines         749      980     +231     
==========================================
+ Hits          708      931     +223     
- Misses         41       49       +8     

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[LasseEngboChr]

Consider avoiding ..1 and ..2 in walk2