update: fix the startup fixture the rollback validation now refuses

The docker-shaped update block in TestUpdateBlockValidatedAtStartup has
source_dir equal to install_dir and no source_rollback, which is exactly the
deployment the new validation refuses. The fixture is meant to be the good
case, so it now says how the source is rolled back.

Found in review of #69.

internal/config/config_test.go

@@ -346,7 +346,8 @@ func TestUpdateBlockValidatedAtStartup(t *testing.T) {
 		"snapshot_dir": "/var/lib/maven/snapshots",
 		"binaries": ["mavend", "mavweb"],
 		"restart_cmd": ["docker", "compose", "up", "-d", "--build", "mavend"],
-		"health_socket": "/run/maven/mavend.sock"
+		"health_socket": "/run/maven/mavend.sock",
+		"source_rollback": "git"
 	}}`
 	c, err := Load(writeConfig(t, good))
 	if err != nil {