Add expiration validation test(s)

[atc0005]

NOTE:

The Go Playground example in the References section below explicitly sets the x509.VerifyOptions.CurrentTime value to aid in testing certificate verification:

	chains, err := leafCert.Verify(x509.VerifyOptions{
		Intermediates: interPool,
		Roots:         rootPool,
		KeyUsages:     []x509.ExtKeyUsage{x509.ExtKeyUsageClientAuth},

		// WARNING: Necessary on Playground. Remove in productive code!
		CurrentTime: time.Date(2019, 12, 18, 0, 0, 0, 0, time.UTC),
	})

I should use a similar approach and add test coverage for certificate expiration.

As part of this, I should search out code directly performing date evaluation and extend (if needed) to allow setting/injecting a known date test value.

References

• https://stackoverflow.com/questions/59383823/how-to-verify-a-certificate-against-an-issuing-chain-in-go
• https://go.dev/play/p/1BNLthzu5Tz