Fix Triangular methods for the case where the diagonal is unity. 

@dmbates pointed out that some of the methods for `Triangular` were broken when the diagonal was set to be unity. This pull request should hopefully fix that.

I have also added some tests and put all the triangular tests into their own file. Eventually I'd like to organize the linalg tests into files corresponding the files in `base/linalg`. This is a small first step.