There are several large chunks of code in mongoc-cluster that aren't being run in any test. Namely, _mongoc_cluster_build_basic_auth_digest, _mongoc_cluster_auth_node_cr, _mongoc_cluster_auth_node_plain, and _mongoc_cluster_auth_node_x509. Look into adding tests for these functions.

See https://codecov.io/gh/mongodb/mongo-c-driver/src/54fbbb7c076128896b4fd2450b7fed2339f1a2f0/src/libmongoc/src/mongoc/mongoc-cluster.c to see a more detailed report on this.