Increase test coverage

[nikosbosse]

Description

This PR increases test coverage by

• adding tests
• removing a bunch of unnecessary test_forecast_type_is... functions. I think we previously used this to automagically determine the forecast type in get_forecast_type(), but that's handled via classes now.

Remaining difference between actual codecov and 100% is due to

• some code in permutation_test() that can't actually ever be reached (I mean it could, but the way we're calling it internally it can't). It could do with some cleaning up, but I'm a tired man...
• the plot_pit() function has a genuine issue (see Testing: Make sure pit() and plot_pit() are correct #359). I didn't want to create a test that's technically working but ultimately wrong

Checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have included the target issue or issues in the PR title as follows: issue-number: PR title
• I have tested my changes locally.
• I have added or updated unit tests where necessary.
• I have updated the documentation if required.
• I have built the package locally and run rebuilt docs using roxygen2.
• My code follows the established coding standards and I have run lintr::lint_package() to check for style issues introduced by my changes.
• I have added a news item linked to this PR.
• I have reviewed CI checks for this PR and addressed them as far as I am able.

[seabbs]

that can't actually ever be reached (I mean it could, but the way we're calling it internally it can't). It could do with some cleaning up, but I'm a tired man...

This is very funny

[nikosbosse]

What can't be reached can't error at least :-D

[seabbs]

Looks good. Missing a saved snapshot