Backport 10703774f0 from the v0.12 branch.
Currently, the test-child-process-spawn-typeerror.js is
calling execFile() on a JavaScript source file, which is
causing failures on Windows. This commit switches to calling
spawn() on an actual executable.
Fixes#8930.
Signed-off-by: Julien Gilli <julien.gilli@joyent.com>