lapack/testlapack: rationalise helpers

[kortschak]

There are a variety of helpers in testlapack that can be used but are not (for example isOrthonormal). I have contributed to this, but it should probably be cleaned up/factored out.

In the case of isOrthonormal the in-line implementations, e.g. in dgelq2.go, do more and have difference tolerances to isOrthonormal, These parameterisations might want to be incorporated into the general.go helper.

[vladimir-ch]

isOrthonormal has been done but is there anything else that you'd like to include here? The contents of general.go and matgen.go could be improved but again, where to stop and where is best to invest time?

[kortschak]

I'm pretty happy with how things are now. The test code is vastly more readable (thank you) and what remains can be done as it's noticed/cared about.

[vladimir-ch]

Does it mean we can close this?