Improve h to cpp to t.cpp switching
Improve h to cpp to t.cpp switching

This adds `u.t.cpp` and `i.t.cpp` as a possible extensions for test files.
Moreover, it is more diligent when searching for possible file locations:

* test files are searched both in current and in test sub-directories
* headers and cpp/cc files are searched in current, in parent, and
  in parent/group directories

steve-downey/dot.emacs,pkryger/exordium,jbibollet/exordium,psipika/exordium,fcassirer/dot.emacs,philippe-grenet/exordium