raco pkg tests: declare a lock for the test

Declare a lock to see whether it has any effect on DrDr, where BC and
CS runs might be competing for a TCP port.

pkgs/racket-test/tests/pkg/test.rkt

@@ -67,7 +67,8 @@
 
 (module+ test
   (module config info
-    (define timeout 3000))
+    (define timeout 3000)
+    (define lock-name "pkg-test"))
   (go))
 
 (module+ main