FindCxxTest: Do not run cxxtestgen through interpreter unless necessary

On platforms that support shebang lines the `cxxtestgen` script should
be able to execute without an explicit interpreter.  Do this if possible
so that the proper interpreter specified by the shebang line is honored.
Ideally we shouldn't even need to find `python` in this case, but that
will be a larger change better made by a dedicated module maintainer.

Closes: #16331