tests: show that the examples still work when directly invoked
tests: show that the examples still work when directly invoked

Since Ic4a124a5cbe2bd24c56e4565d27d313fe4da703f, there was no automated
test which would check if the generated examples *really* work. When I
was playing with this, I managed to break it at least once (especially
when working on overriding sys.args, i.e.,
I53833a5513abae0abd57065a49c0f357890e0820).

This now requires an equivalent of `pip install` before the tests can be
run.

Change-Id: I595a3efe29b3ee13800c5cb71f28a5f370988617

Telecominfraproject/gnpy