Disable retryable handshakes by default for mock server tests

[eramongodb]

There have been several mock server tests since #1141 was merged that required manually disabling retryable handshakes to maintain correct test behavior (i.e. expectations that a single mock server hangup request should trigger progress). This PR removes those workarounds in favor of setting retryReads=false and retryWrites=false by default in the URI returned by mock_server_get_uri and mock_rs_get_uri. Mock server tests that actually require retryable handshakes are thus appropriately limited to test-mongoc-retryable-reads.c and test-mongoc-retryable-writes.c where retryReads=true or retryWrites=true is explicitly set as needed.