tests: virsh-output: Mark as expensive

The test takes more than a second on a beefy machine. While it's more
useful than some expensive tests it's not worth running all the time.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>

tests/virsh-output

@@ -2,6 +2,8 @@
 
 . "$(dirname $0)/test-lib.sh"
 
+test_expensive
+
 if [ ! -x /bin/bash ]; then
     echo skipping test - no bash
     exit 77