I did the following:
1. add a testmod source set (probably could use test)
2. a configurations to extend from main
```
configurations {
    testmodCompile.extendsFrom(compile)
    testmodCompileOnly.extendsFrom(compileOnly)
    testmodRuntimeOnly.extendsFrom(runtimeOnly)
}
```
3. add the module to run config
```
// In minecraft.runs.(client, server, and optionally data).mods
mainmod_modid {
  source sourceSets.main
}
testmod_modid {
  source sourceSets.testmod
}
```
4. refresh gradle + genIntellijRuns