[build] Avoid rebuilding of test-internal.c
[build] Avoid rebuilding of test-internal.c

The tests under src/adg-gtk/tests share the code in test-internal.[ch]
with src/adg/tests. Using LDADD on the object file to include this code
instead of adding a dependency on the source file, because the latter
will rebuild test-internal.c in the current builddir.

ntd/adg,ntd/adg