When resmoke.py creates the string for the --eval argument for mongo, it builds the string in the following order.

1. List of global variables
2. The "eval" field from YAML config

In the mongo-tools project, there is a need to override servers.js and servers_misc.js. In order to do this though, the loads for these files must proceed the global variable setting in the eval argument.

The tools do this via a new field called "eval_prepend":
See https://github.com/mongodb/mongo-tools/blob/a8800cf352f1efd5f442764a151586d1701dc9a2/test/qa-tests/buildscripts/resmokelib/core/programs.py#L162-L163

An example suite that uses this is the core suite: https://github.com/mongodb/mongo-tools/blob/a8800cf352f1efd5f442764a151586d1701dc9a2/test/qa-tests/buildscripts/resmokeconfig/suites/core.yml#L18