Change test build setup to allow one test per object type

This changes the build setup for the tests, so we can have one test
file per object type, which was not possible before:

    tests/check/objects/ws_object - for ws_object type
    tests/check/objects/ws_set - for ws_set type
    tests/check/objects/ws_array - for ws_array type

And so on.

The appropriate cmake files for the single tests have to be added by
the author of the related test source file. Also, the entry for the
subdirs for everything else then "ws_object" have to be added by the
test authors.

---

This setup should also allow a several-file test for one type.

---

@neithernut, @TheNeikos please review carefully, you guys know that
I'm not so good with cmake.