Fix test cases by putting function around expressions

syntax/basic_type_as_constructor.invalid.mj

 class BasicTypeAsConstructor {
-    new int();
+    public int constructInt() {
+        return new int();
+    }
 }

syntax/pre_incr_vs_pre_decr.invalid.mj

 class PreDecrementIsNotWhatItSeems {
-    --thisIsTotallyFine - !!!++butThisIsNot;
+    public void controversial() {
+        --thisIsTotallyFine - !!!++butThisIsNot;
+    }
 }

syntax/primary_expr_as_field_access.invalid.mj

 class PrimaryExprAsFieldAccess {
-    1234.null;
+    public void nullIsAField() {
+        1234.null;
+    }
 }