[1.8.x] Fixed #24289 -- Reversed usage of Field.many_to_one and one_to_many.

Thanks Carl Meyer and Tim Graham for the reviews and to all involved
in the discussion.

Backport of 18c0aaa912 from master

tests/model_fields/test_field_flags.py

@@ -31,22 +31,22 @@ RELATION_FIELDS = (
     GenericRelation,
 )
 
-ONE_TO_MANY_CLASSES = {
+MANY_TO_MANY_CLASSES = {
+    ManyToManyField,
+}
+
+MANY_TO_ONE_CLASSES = {
     ForeignObject,
     ForeignKey,
     GenericForeignKey,
 }
 
-MANY_TO_ONE_CLASSES = {
+ONE_TO_MANY_CLASSES = {
     ForeignObjectRel,
     ManyToOneRel,
     GenericRelation,
 }
 
-MANY_TO_MANY_CLASSES = {
-    ManyToManyField,
-}
-
 ONE_TO_ONE_CLASSES = {
     OneToOneField,
 }