rewrite tests to use sub-tests, improve error-handling in tests, and use t.Cleanup()

[thaJeztah]

• [x] depends on https://github.com/docker/docker-credential-helpers/pull/278

See individual commits for details

[codecov-commenter]

Codecov Report

Patch coverage: 57.14% and project coverage change: -0.20 :warning:

Comparison is base (9ff5b61) 55.55% compared to head (59cb6ec) 55.35%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #275 +/- ## ========================================== - Coverage 55.55% 55.35% -0.20% ========================================== Files 9 9 Lines 666 672 +6 ========================================== + Hits 370 372 +2 - Misses 253 257 +4 Partials 43 43 ``` | [Impacted Files](https://app.codecov.io/gh/docker/docker-credential-helpers/pull/275?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=docker) | Coverage Δ | | |---|---|---| | [osxkeychain/osxkeychain\_darwin.go](https://app.codecov.io/gh/docker/docker-credential-helpers/pull/275?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=docker#diff-b3N4a2V5Y2hhaW4vb3N4a2V5Y2hhaW5fZGFyd2luLmdv) | `69.29% <57.14%> (-1.79%)` | :arrow_down: |

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[thaJeztah]

This one should be ready for review now as well; @crazy-max PTAL ❤️