Do not run prediff on mandelbrot test(s).

Mandelbrot outputs a binary file (the 0s and 1s), so grep is not a suitable tool for filtering it.

git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@23644 3a8e244f-b0f2-452b-bcba-4c88e055c3ca