Fix broken eqErrGRPC helper function in integration tests

[zhijun42]

The test helper function eqErrGRPC has always been flawed since introduced in 2016.

func eqErrGRPC(err1 error, err2 error) bool {
	return !(err1 == nil && err2 != nil) || err1.Error() == err2.Error()
}

When the first argument was non-nil (usually the case in tests), it will always return true. This has made all error assertions using this helper effectively no-ops.

Changes:

• Replace broken implementation with correct gRPC status comparison using status.Convert()
• Add unit tests covering code/message permutations and edge cases
• Fix test bugs in v3_auth_test.go exposed by the fix (ErrUserEmpty → ErrGRPCUserEmpty)
• Add documentation explaining gRPC error wire serialization behavior and the correct usage.

[k8s-ci-robot]

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[k8s-ci-robot]

Hi @zhijun42. Thanks for your PR.

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[serathius]

This function is badly designed, it promises too much, validates too little. Instead of writing a essay on how to use a test function, just panic if user provides incorrect input.

[zhijun42]

Good point! Refactored to just panic. And removed my essay.

[serathius]

Instead of writing comments improve function name.

[zhijun42]

I can't come up with a better name.

• matchGRPCErrors
• errorsEqualByGRPCStatus
• mustEqualGRPCErrors
• mustGRPCErrorsEqual
• verifyGRPCErrorsEqual
• requireGRPCErrorsEqual
• validateAndCompareGRPCErrors
• grpcStatusCodeAndMessageEqual