Motivation
DNS tests are run as part of separate tasks:

• test-dns-openssl
• test-dns-auth-openssl
• test-dns-loadbalanced-openssl
• test-dns-darwinssl
• test-dns-auth-darwinssl
• test-dns-winssl
• test-dns-auth-winssl

And they are run in Evergreen in a circuitous way.

• The Evergreen "run tests" function expects a DNS Evergreen expansion to be set to on, loadbalanced, or dns-auth.
• The run-tests.sh script expects a DNS environment variable of the same values.
• run-tests.sh sets MONGOC_TEST_DNS=on or MONGOC_TEST_DNS_LOADBALANCED=on to enable DNS tests in test-libmongoc.
• Enabling MONGOC_TEST_DNS=on expects a replica set to be running, and MONGOC_TEST_DNS_LOADBALANCED=on expects a sharded cluster with load balancer to be running.
• The separate test-dns-... tasks only run tests matching the path /dns/*

This seems like an unnecessarily complicated special case for testing.

Scope

• Remove the test-dns-* tasks, and run the DNS tests with the rest of the test suite.
• Remove the MONGOC_TEST_DNS and MONGOC_TEST_DNS_LOADBALANCED environment variables.
• Add skips of DNS tests within test-libmongoc where applicable (e.g. if libresolv is unavailable, or if the topology does not match what is necessary for the tests).