test: move common tls connect setup into fixtures

TLS connection setup boilerplate is common to many TLS tests, factor it
into a test fixture so tests are clearer to read and faster to write.

PR-URL: https://github.com/nodejs/node/pull/10389


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>