feat: Add test utilities

[nieomylnieja]

Motivation

Testing validation defined with govy can be a daunting task.
Govy structured errors are information rich, while this is great for end users, it can be a tedious task to verify if one govy.ValidatorError is equal to another govy.ValidatorError.
Often times we might not care about fields like description or even message, we might just want to verify error codes for given properties.

Govy's goal is to not only make the end-user's life better but the programmer's just as well, it would benefit the second party to have a ready-to-use utility which could make the testing process of govy-defined validation a breeze.

Summary

• Added govytest package.
• Added tests to some of the internal helpers.
• Added new builtin rule OneOfProperties which ensures that at least one of the properties provided by getters is set.

Release Notes

Added govytest package which exposes utilities which help test govy validation rules.
It comes with two functions AssertNoError, which ensures no error was produced, and AssertError which checks that the expected errors are equal to the actual govy.ValidatorError.
Added OneOfProperties rule which checks if at least one of the properties is set.

[github-actions]

Test coverage changes:

Package	Before	After	Diff
github.com/nobl9/govy/internal/examples	30%	55%	✔️
github.com/nobl9/govy/internal/nameinfer	15%	16%	✔️
github.com/nobl9/govy/pkg/govy	84%	86%	✔️
github.com/nobl9/govy/pkg/govyconfig	68%	63%	❌
github.com/nobl9/govy/pkg/rules	83%	79%	❌