This commit adds a test case for the QubesVM class's is_fully_usable
method. The verified scenarios are as follows:
* The VM has qrexec enabled, and the qrexec service has been
successfully started.
(The VM becomes "fully usable" in this case.)
* The VM has qrexec enabled, and the qrexec service has failed to start
(Error handling case; the VM is *not* fully usable.)
* The VM does *not* have qrexec enabled.
(The VM becomes "fully usable" in this case.)
5d755eb065