tests: Clarify the interface might not be on the host

We try to delete the interface, but it lot of tests it won't be there
unless we failed to move it to the container. Let's just clarify that in
a comment and redirect the error output to /dev/null, as it seems an
error otherwise while it is completely normal.

Signed-off-by: Rodrigo Campos <rodrigo@amutable.com>

tests/integration/netdev.bats

@@ -35,7 +35,10 @@ function setup() {
 }
 
 function teardown() {
-	ip link del dev dummy0
+	# The interface might be on the host or not, depending the test. If it's on the host, let's
+	# delete it.
+	ip link del dev dummy0 2>/dev/null
+
 	delete_netns
 	teardown_bundle
 }