Fixed python3 detection for spirv-cross-test.
Fixed python3 detection for spirv-cross-test.

Windows Python 3.x installations do not include a python3.exe; this
caused the old test to fail and issue a spurious warning.

KhronosGroup/SPIRV-Cross,h3xl3r/SPIRV-Cross,KhronosGroup/SPIRV-Cross,h3xl3r/SPIRV-Cross,h3xl3r/SPIRV-Cross,h3xl3r/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross,h3xl3r/SPIRV-Cross,KhronosGroup/SPIRV-Cross,KhronosGroup/SPIRV-Cross