- tests.test_setlinkrev: adjusted testcases for the new set_link_rev semantics (see #72)
- tests/test_setlinkrev: added two new testcases

Behave correctly if the _link already has a rev attribute.

Also added a fixture file that I forgot in
commit 6b579791a110bfefc79c4d310ff394c2bfe5c26f.
- do_setlinkrev, set_link_rev: fixed #72

Also refactored set_link_rev code a bit so that the new _set_link_rev
function could be used by link_pac in the future.