Issue 553: interval_coverage_ testing improvements

[seabbs]

Description

This PR closes #553

As it says on the tin adds some safety unit tests

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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (545b40c) 83.71% compared to head (85733bf) 84.30%.

❗ Current head 85733bf differs from pull request most recent head 520785d. Consider uploading reports for the commit 520785d to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #554      +/-   ##
===========================================
+ Coverage    83.71%   84.30%   +0.58%     
===========================================
  Files           22       21       -1     
  Lines         1726     1720       -6     
===========================================
+ Hits          1445     1450       +5     
+ Misses         281      270      -11     

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[nikosbosse]

Looks good, thank you!