.travis.yml: Functional test for non existent tests

Avocado should handle gracefully and not crash.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

.travis.yml

@@ -18,3 +18,4 @@ script:
     - make -C docs html 2>&1 | grep -E '(ERROR|WARNING)' || test $? -eq 1
     - ./scripts/avocado run "sleeptest sleeptest"
     - ./scripts/avocado run "sleeptest failtest sleeptest" || test $? -eq 1
+    - ./scripts/avocado run "bogustest" || test $? -ne 3