Make SSL tests more robust

Make SSL tests more robust

Someone running these test could have key or certificate files in
their ~/.postgresql/, which would interfere with the tests.  The way
to override that is to specify sslcert=invalid and/or
sslrootcert=invalid if no actual certificate is used for a particular
test.  Document that and fix up one test that had a risk of failing in
these circumstances.

Discussion: https://www.postgresql.org/message-id/flat/398754d8-6bb5-c5cf-e7b8-22e5f0983caf@2ndquadrant.com/