CDRIVER-5645 skip auth tests running against MongoDB 3.6.

[kevinAlbs]

Summary

Skip authentication tests running against 3.6 servers.

Verified by this patch

Background & Motivation

CDRIVER-4815 dropped support for 3.6 servers. Some test servers are running 3.6, resulting in task failures. Example:

Ping failure: Server at <REDACTED:auth_host>:27017 reports wire version 6, but this version of libmongoc requires at least 7 (MongoDB 4.0)

DEVPROD-9029 tracks upgrading test servers. This PR proposes skipping tests in the short-term as they are expected to fail.

[eramongodb]

Style suggestion; otherwise, LGTM.

[eramongodb]

Recommend using {} || {} instead of if/else to better distinguish the conditional blocks from existing code:

{
  # Skip 3.6. Remove this block when resolving CDRIVER-5645.
  echo "Skipping tests on server 3.6 (using 'auth_host' or 'auth_gssapi') until DEVPROD-9029 is addressed."
} || {
# FIXME: CDRIVER-2008 for the cygwin check
if [[ "${OSTYPE}" != "cygwin" ]]; then
  echo "Authenticating using X.509"
  ...
fi
}