Fix make plugin clean

Author: nygrenh

tmc-langs-make/src/main/java/fi/helsinki/cs/tmc/langs/make/MakePlugin.java

@@ -233,7 +233,7 @@ private boolean builds(Path dir) {
 
     @Override
     public void clean(Path path) {
-        String[] command = {"make", "test"};
+        String[] command = {"make", "clean"};
         ProcessRunner runner = new ProcessRunner(command, path);
 
         try {