test: Ensure replication connection can be formed

[cjfinnell]

This test mostly demonstrates that out-of-the-box replication connection
is only supported on postgres images > 9. As we are currently running
12.6 in production, this shouldn't be an issue for actual use.

Test output:

--- PASS: TestReplicationConnect (0.00s)
    --- PASS: TestReplicationConnect/psql-int-test:11-a9b5bca (13.42s)
    --- PASS: TestReplicationConnect/psql-int-test:12-a9b5bca (13.68s)
    --- PASS: TestReplicationConnect/psql-int-test:10-a9b5bca (13.93s)
    --- PASS: TestReplicationConnect/postgres:9 (14.13s)
    --- PASS: TestReplicationConnect/postgres:11 (14.31s)
    --- PASS: TestReplicationConnect/postgres:12 (14.53s)
    --- PASS: TestReplicationConnect/psql-int-test:9-a9b5bca (14.69s)
    --- PASS: TestReplicationConnect/postgres:10 (14.84s)
--- PASS: TestVersionMigration (0.00s)
    --- PASS: TestVersionMigration/custom11To11 (23.41s)
    --- PASS: TestVersionMigration/9.5To9.6 (26.15s)