Added support for custom TextTestResult class.

Hello,

This small patch adds support for custom result classes, similarly to how pyut TextTestResult does since 2.7.2.

(Disclaimer: this work was done while at Cisco Systems LTD and all the required authorizations for pushing upstream were obtained through the internal open source patch channel)
Shouldn't this be on both paths, and not just the >= 2.7.2 one?  Otherwise, `_makeResult()` would create the wrong kind of result in the < 2.7.2 case, no?