Stop using atf::application::app in test programs

The app class is a pretty horrible mess, plain and simple.  Stop using
it so that there are fewer layers of indirection between the invocation
of the program and the execution of the test case code.
Stop catching unhandled exceptions in atf-c++

Let the caller receive an abort trap when an exception propagates out of
the program so that he can deal with it at wish: e.g. by inspecting it with
a debugger and getting proper details about the fault point.
Merge branch 'no-catch-unhandled-exceptions'

Fixes #7.