Allow rtesting to use custom diff func

[scothis]

Test cases can now specify a custom diff function which can be used to either use custom cmp options, or replace cmp with some other differ.

The existing set of cmp options are matched using the DiffReason. Custom funcs should assume additional DiffReasons will be defined in the future. The DefaultDiff func is used if a custom Diff is not defined. Custom funcs may delegate to the default for DiffReasons they choose not to modify.

Refs #589

[scothis]

I'm unsure this is the best approach to fully decouple the existing behavior without either coupling the public API to cmp, or introducing each as a custom func.

[mamachanko]

An alternative approach could be Differ interface with a default implementation where each "diff reason" maps to a method, e.g. Differ#DiffRaw(actual, expected) string, Differ#DiffTrackRequest(actual, expected) string etc. We'd get a bit of method sprawl, but it may be easier to provide a custom differ without accidentally missing a diff reason. At the same time, maintainers need to take care to always call the right diff method when asserting test case. To some degree that's already the case, what with the different opts / reasons.

type X509CertificateDiffer struct {
	rtesting.DefaultDiffer
}

func (d *X509CertificateDiffer) DiffStashedValue(expected, actual interface{}) string {
	return cmp.Diff(expected, actual, cmp.AllowUnexported(big.Int{}))
}

var _ rtesting.Differ = X509CertificateDiffer{}

[codecov]

Codecov Report

Attention: Patch coverage is 58.42697% with 37 lines in your changes missing coverage. Please review.

Project coverage is 58.22%. Comparing base (df0175b) to head (3544af8).

Files with missing lines	Patch %	Lines
testing/diff.go	65.51%	20 Missing ⚠️
testing/subreconciler.go	0.00%	7 Missing ⚠️
testing/reconciler.go	0.00%	5 Missing ⚠️
testing/webhook.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #590      +/-   ##
==========================================
- Coverage   58.26%   58.22%   -0.04%     
==========================================
  Files          33       34       +1     
  Lines        3800     3873      +73     
==========================================
+ Hits         2214     2255      +41     
- Misses       1492     1524      +32     
  Partials       94       94              

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[mamachanko]

I think this should work.

However, Diff is starting to compete with testing#SubReconcilerTestCase's
Verify and VerifyStashedValue and testing#ReconcilerTestCase's Verify field. Their propositions are muddling a bit, aren't they? Afaik Diff should be able to accomplish everything that Verify* is designed to do.

Maybe we should start deprecating Verify* in favour of Diff.

[mamachanko]

An alternative approach could be Differ interface with a default implementation where each "diff reason" maps to a method, e.g. Differ#DiffRaw(actual, expected) string, Differ#DiffTrackRequest(actual, expected) string etc. We'd get a bit of method sprawl, but it may be easier to provide a custom differ without accidentally missing a diff reason. At the same time, maintainers need to take care to always call the right diff method when asserting test case. To some degree that's already the case, what with the different opts / reasons.

type X509CertificateDiffer struct {
	rtesting.DefaultDiffer
}

func (d *X509CertificateDiffer) DiffStashedValue(expected, actual interface{}) string {
	return cmp.Diff(expected, actual, cmp.AllowUnexported(big.Int{}))
}

var _ rtesting.Differ = X509CertificateDiffer{}