Fix failing test_find_program test (Windows-only)
Fix failing test_find_program test (Windows-only)

Skip finding a .py script w/o extension on Windows if `.PY` isn't in
PATHEXT; closes #4355

mesonbuild/meson,becm/meson,mesonbuild/meson,pexip/meson,jpakkane/meson,becm/meson,jpakkane/meson,becm/meson,becm/meson,pexip/meson,pexip/meson,jpakkane/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,mesonbuild/meson,QuLogic/meson,jpakkane/meson,jpakkane/meson,mesonbuild/meson,mesonbuild/meson,pexip/meson,pexip/meson,becm/meson,QuLogic/meson,jpakkane/meson,becm/meson,QuLogic/meson,becm/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,becm/meson,mesonbuild/meson,jpakkane/meson,becm/meson,jpakkane/meson,becm/meson,mesonbuild/meson,pexip/meson,pexip/meson,jpakkane/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson