Merge GVariant variable arguments support and put it under tests.

Also, remove the hack of the test case directly '#include'ing .c files
from glib/.  Instead, create a non-installed gvariant-internal.h that
the tests can include and make the symbols in it visible on the symbol
table of the shared library.  These symbols (as they are present in no
installed header files) are not part of the API of GLib.

Increase test coverage in a few other areas.