Add test.Err and clean up tests

[FollowTheProcess]

Description

Adds a public function test.Err that fails if the provided err is equal to nil. effectively the opposite of test.Ok

Also refactored the tests to be neater and call shouldPass or shouldFail in a loop

Motivation and Context

Closes #

How Has This Been Tested?

Checklist

• [ ] I have updated the documentation if needed.
• [x] I have updated the tests if needed.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (fc3bb0c) 100.00% compared to head (a7d18d1) 100.00%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 1 1 Lines 50 55 +5 ========================================= + Hits 50 55 +5 ``` | [Files Changed](https://app.codecov.io/gh/FollowTheProcess/test/pull/2?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tom+Fleet) | Coverage Δ | | |---|---|---| | [test.go](https://app.codecov.io/gh/FollowTheProcess/test/pull/2?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Tom+Fleet#diff-dGVzdC5nbw==) | `100.00% <100.00%> (ø)` | |

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