Unit tests for the buffer output stream

The unit tests compiles, but doesn't link. I'll see if adding the file
loader code helps. So for now it is disabled.
GtkSourceFileLoader (compilable)
build: compile all the files in the private .la

There are problems for linking the buffer-output-stream unit tests.

- The unit tests cover gtksourcebufferoutputstream.c.
- gtksourcebufferoutputstream.c is built in libgtksourceview-private.la.
- gtksourcebufferoutputstream.c uses a private function built in
  libgtksourceview-3.0.la (_gtk_source_buffer_set_as_invalid_character()).

I get this error when linking the unit tests:
undefined reference to `_gtk_source_buffer_set_as_invalid_character'

It seems that the problem is that the private.la is not self-contained.
By compiling _all_ the files in the private.la, it works. The real
library just links the private.la and other stuff, without compiling
anything.
Enable unit tests for the BufferOutputStream

Now the unit tests link fine, and 'make check' passes fine too.