Make tests pass on CI

Jenkins matrix builds almost never check out a project to a directory matching the name of the repository, so Yeoman's reliance on the path following the generator-* pattern doesn't work so well.

This PR does 2 things to improve testing on CI:
 * makes use of the 2 argument form of `Environment.prototype.register()` to specify the path as well as the name of the generator
 * provides an implementation of `Must.prototype.members()`, which seems to be non-standard.