Fix the failing (internal) Key Vault cert-related tests.

The Get-X509FromSecret helper function was making the wrong assumption that the
leaf certificate in a chain would be located at the beginning of the secret
returned by the Key Vault service. Some cert-related tests started to fail when
the Key Vault service changed the ordering so that the leaf certificate is now
located at the end of the secret. This was not a breaking change, as the Key
Vault service does not guarantee any ordering of the certs in the chain.

To address this issue, this change instead searches the entire collection of
certs in the secret by subject distinguished name (we replace the
Get-X509FromSecret helper with Get-X509FromSecretBySubjectDistinguishedName).
This increases test resiliency by not relying on any cert ordering.

This change also extends the NotAfter property of a hardcoded test cert that 3
tests rely on by 1000 years, since there were test failures due to the cert
having expired in October 2016.

Author: adarce