Use Shebang in cxxtestgen when appropriate (#3458)

Use Shebang in cxxtestgen when appropriate (#3458)

CxxTest's generator has one of three names, depending on the CxxTest
version:
* `cxxtestgen.py`, a python program
* `cxxtestgen`, some kind of program with a shebang
* `cxxtestgen.pl`, a perl program

We were mistakenly assuming that the `cxxtestgen` form was always a
python program.

Now, if we find that form on the system, we assume that it is has a
shebang, and is executable.