gst_hacks.py compatibility with OS X

Dynamic libraries are ".dylib" not ".so" on OS X.

If I only do `ctypes.CDLL("libgstreamer-1.0.dylib")`, `dlopen` won't
find the library in `/opt/local/lib` (where GStreamer is installed by
the "macports" package manager), even though I'm using
`/opt/local/bin/python`. Adding `/opt/local/lib` to `DYLD_LIBRARY_PATH`
doesn't work either, I get all sorts of "Symbol not found" errors. So
I'm calculating the path from where GStreamer's GObject-Introspection
typelib data is installed. I've only tested this with macports.
Revert "(FAILING TEST) Don't skip test_save_video on OS X"

This reverts commit 0e913337b646297cfc91b626c3635f9ff0ca8574.
IN PROGRESS: Fix test_wait_for_match weird interlacing issue