Attempt to fix flakey test

This test checks that we don't leak memory, but it sometimes timeout on
the CI. Doing an explicit GC step after every iteration might help.

Author: smarter

tests/run/module-serialization-proxy-class-unload.scala

@@ -14,6 +14,7 @@ object Test {
       // This would "java.lang.OutOfMemoryError: Java heap space" if ModuleSerializationProxy
       // prevented class unloading.
       deserializeDynamicLoadedClass()
+      System.gc()
     }
   }