[tlscommon] Add test helper methods

[khushijain21]

What does this PR do?

This PR moves some helper methods for testing in a test_helper.go file to be able to reuse these methods publicly.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

[mauri870]

I confirmed that the functions are now properly exported.

$ go doc ./transport/tlscommon | grep 'GetCertFingerprint\|GenTestCerts'
func GenTestCerts(t *testing.T) map[string]*x509.Certificate
func GetCertFingerprint(cert *x509.Certificate) string

[khushijain21]

/test

[elasticmachine]

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