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 (closed)