Refine test assertion output

[scothis]

• allow colorized diffs, use COLOR_DIFF=true to force on
• clarify what is unexpected or missing
• reference specific expected field and index when appropriate
• hide "for config X" for the default config, continue to show for extra configs

[codecov[bot]]

Codecov Report

Patch coverage: 79.59% and project coverage change: +0.11% :tada:

Comparison is base (a688686) 60.78% compared to head (3be702e) 60.89%.

:exclamation: Current head 3be702e differs from pull request most recent head f98799a. Consider uploading reports for the commit f98799a to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #423 +/- ## ========================================== + Coverage 60.78% 60.89% +0.11% ========================================== Files 25 26 +1 Lines 2494 2514 +20 ========================================== + Hits 1516 1531 +15 - Misses 893 896 +3 - Partials 85 87 +2 ``` | [Files Changed](https://app.codecov.io/gh/vmware-labs/reconciler-runtime/pull/423?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vmware-labs) | Coverage Δ | | |---|---|---| | [testing/reconciler.go](https://app.codecov.io/gh/vmware-labs/reconciler-runtime/pull/423?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vmware-labs#diff-dGVzdGluZy9yZWNvbmNpbGVyLmdv) | `0.00% <0.00%> (ø)` | | | [testing/subreconciler.go](https://app.codecov.io/gh/vmware-labs/reconciler-runtime/pull/423?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vmware-labs#diff-dGVzdGluZy9zdWJyZWNvbmNpbGVyLmdv) | `0.00% <0.00%> (ø)` | | | [testing/webhook.go](https://app.codecov.io/gh/vmware-labs/reconciler-runtime/pull/423?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vmware-labs#diff-dGVzdGluZy93ZWJob29rLmdv) | `0.00% <0.00%> (ø)` | | | [testing/color.go](https://app.codecov.io/gh/vmware-labs/reconciler-runtime/pull/423?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vmware-labs#diff-dGVzdGluZy9jb2xvci5nbw==) | `81.25% <81.25%> (ø)` | | | [testing/config.go](https://app.codecov.io/gh/vmware-labs/reconciler-runtime/pull/423?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=vmware-labs#diff-dGVzdGluZy9jb25maWcuZ28=) | `86.93% <92.85%> (-0.62%)` | :arrow_down: |

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