[tests] Fix certificate tests after Apple started blocking the Thawte…

… certificate. (xamarin#19650)

The “Thawte SGC CA” certificate we're using for testing is now blocked by
Apple, evaluating trust results in this error:

> “Thawte SGC CA” certificate is blocked

So update the mail_google_com certificates to what mail.google.com serves now, and also update the corresponding parent certificates (which have changed).

Then adjust the tests accordingly.

Finally delete all unused certificates from the code.

Fixes these test failures:

    MonoTouchFixtures.Security.TrustTest
        [FAIL] Trust_FullChain :   GetTrustResult-2
            Expected: RecoverableTrustFailure
            But was:  FatalTrustFailure
               at MonoTouchFixtures.Security.TrustTest.Trust_FullChain(SecTrust trust, SecPolicy policy, X509CertificateCollection certs) in /Users/builder/azdo/_work/1/s/xamarin-macios/tests/monotouch-test/Security/TrustTest.cs:line 389
               at MonoTouchFixtures.Security.TrustTest.Trust_FullChain() in /Users/builder/azdo/_work/1/s/xamarin-macios/tests/monotouch-test/Security/TrustTest.cs:line 321
        [FAIL] Trust2_FullChain :   GetTrustResult-2
            Expected: RecoverableTrustFailure
            But was:  FatalTrustFailure
               at MonoTouchFixtures.Security.TrustTest.Trust_FullChain(SecTrust trust, SecPolicy policy, X509CertificateCollection certs) in /Users/builder/azdo/_work/1/s/xamarin-macios/tests/monotouch-test/Security/TrustTest.cs:line 389
               at MonoTouchFixtures.Security.TrustTest.Trust2_FullChain() in /Users/builder/azdo/_work/1/s/xamarin-macios/tests/monotouch-test/Security/TrustTest.cs:line 449