The snmp and snmp_WT tasks both have their own resmoke.py invocation separate from the "run tests" function. To avoid needing to update multiple places in the Evergreen project configuration—for example, when adding a new environment variable—we should rewrite these task definitions to use the "run tests" function. We can use an approach similar to the "do multiversion setup" that was added as part of SERVER-21487.

After this change, the only way to invoke resmoke.py in the etc/evergreen.yml project configuration will be through the "run tests" function. This also helps to consolidate the codepaths where we'd otherwise need to manage Python dependencies installed a virtualenv, the math for the number of resmoke.py --jobs to use, etc.