test_filters: move from tests/unit to lib/filter/tests

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
rewrite/rewrite-set-tag: split off from rewrite-expr

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
rewrite/rewrite-set: split off from rewrite-expr

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
rewrite/rewrite-subst: split off from rewrite-expr

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>
rewrite: clean up includes

Because of the split a number of includes remained lingering, clean them
up.

Signed-off-by: Balazs Scheidler <bazsi@balabit.hu>