[CMake] Running CxxTest fails on Fedora 24

[Zan Lynx]

The Fedora /usr/bin/cxxtestgen script calls /usr/bin/python3. NOT
python2 or python.

CMake runs /usr/bin/python /usr/bin/cxxtestgen which is linked to
python2, and the script fails.

I don't quite understand why CMake feels the need to call it explicitly
with the Python interpreter that it found instead of using the shebang.
Maybe Windows? Eh.

I am fixing it in my script by doing this:

> FIND_PACKAGE(PythonInterp 3 QUIET)
> FIND_PACKAGE(CxxTest)

I thought this might help anyone else hitting this.

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