test/README.md: mention mkosi genkey

This is a required first step before testing and it's not specifically
mentioned in the doc.  This tripped me up for a while, so let's save the
trouble for the next person.

test/integration-tests/README.md

@@ -8,6 +8,12 @@ latest version of mkosi. See
 for more specific details. Make sure `mkosi` is available in `$PATH` when
 reconfiguring meson to make sure it is picked up properly.
 
+If you haven't done it already, be sure to generate signing keys for `mkosi`:
+
+```shell
+$ mkosi genkey
+```
+
 Next, we can build the integration test image with meson:
 
 ```shell