tests/virtconvtest: skip test if unar is missing

If you are running all tests, we should properly skip the test, if something
is missing instead of silently ignore that tests.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>

(crobinso: make it skip the test, rather than raise an exception)

tests/virtconvtest.py

@@ -68,7 +68,7 @@ class TestVirtConv(unittest.TestCase):
 
         if (os.path.splitext(in_path)[1] in [".zip"] and
             not find_executable("unar")):
-            return
+            self.skipTest("Install 'unar' to run all tests.")
 
         try:
             os.chdir(os.path.dirname(in_path))