The transactions tests with useMultipleMongoses: true currently is implemented in our test runner by hard coding the hosts "localhost:27017" and "localhost:27018". We should provide a way to override that with environment variables, to be consistent with the rest of the test runner.

The transactions_test_before_test and transactions_test_after_test in test-mongoc-transactions.c also appears to hard-code URIs in their calls to _reset_server and _disable_failpoints, respectively. This is done regardless of whether useMultipleMongoses is set, so it also applies to RS environments.