Update tests to not use `reflect.DeepEqual` on errors

[anderseknert]

Hacking on the OPA repo in VS Code and I'm seeing a bunch of warnings from the language server about us using reflect.DeepEqual to compare errors. This is apparently problematic for... reasons, as described here and elsewhere. Don't have time to spend on this tonight, but noting it down so that we can fix this and enable whatever linter is flagging this issue, as it seems legit to me.

[stale]

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