Add additional tests and fallback logic when covariate is degenerate

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 84.54%. Comparing base (adeb867) to head (f5e3762).

Files	Patch %	Lines
src/mozanalysis/frequentist_stats/linear_models.py	94.44%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #229 +/- ## ========================================== + Coverage 84.45% 84.54% +0.09% ========================================== Files 24 24 Lines 1267 1281 +14 ========================================== + Hits 1070 1083 +13 - Misses 197 198 +1 ``` | [Flag](https://app.codecov.io/gh/mozilla/mozanalysis/pull/229/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla) | Coverage Δ | | |---|---|---| | [project](https://app.codecov.io/gh/mozilla/mozanalysis/pull/229/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla) | `84.54% <94.44%> (+0.09%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=mozilla#carryforward-flags-in-the-pull-request-comment) to find out more.

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