tests: Move nominatim-*.json to tests directory

So they are with the other test JSON files. This also removes the need
for TEST_SRCDIR — we can now use g_test_build_filename() instead, which
uses the G_TEST_SRCDIR/G_TEST_BUILDDIR environment variables which will
eventually be passed in by glib-tap.mk (in following commits).

https://bugzilla.gnome.org/show_bug.cgi?id=772928