Add vc-backend and vc-responsible-backend tests

Add vc-backend and vc-responsible-backend tests

* lisp/vc/vc-hooks.el (vc-file-setprop, vc-file-getprop)
(vc-file-clearprops): Use properties on absolute files.

* test/lisp/vc/vc-tests.el (vc-test--unregister-function):
Clear file properties.
(vc-test--register): Add tests for `vc-backend' and
`vc-responsible-backend'.  Catch other errors but `vc-not-supported'.
(vc-test--state, vc-test--checkout-model): Catch other errors
but `vc-not-supported'.
(vc-test--working-revision): Fix test for RCS and SCCS.  Catch
other errors but `vc-not-supported'.
(vc-test-src02-state): Mark as an expected failure.