Add run-js-test executable to run tests

ST makes use of GTK+ for input methods and for icon themes; therefore
we have need to initialize GTK+ in order to test these parts of Clutter.

Instead of LD_PRELOADING our module, use a separately compiled executable
that links to the UI components in GNOME Shell, initializes Clutter and
GTK+ and hooks them together.

Getting all the symbols from St and the GUI components exported for
use via GJS requires a bit of contortion: we need to actually link the
St convenience library into a shared library and link the executable
to that since there is no way with libtool to take a convenience library
and put all its symbols into an executable --whole-archive style.

https://bugzilla.gnome.org/show_bug.cgi?id=633657