[test][core] Add proper support for testing the fork command

It will ensure it succeeds if the type can be forked and fail
otherwise.

beat/cmdline/test/core.py

@@ -142,14 +142,17 @@ class AssetLocalTest:
         obj = self.object_map["create"]
         obj2 = self.object_map["fork"]
         self.test_create(obj)
-        exit_code, outputs = self.call("fork", obj, obj2, prefix=tmp_prefix)
-        nose.tools.eq_(exit_code, 0, outputs)
-        s = self.storage_cls(tmp_prefix, obj2)
-        nose.tools.assert_true(s.exists())
-
-        # check fork status
         with common.Selector(tmp_prefix) as selector:
-            nose.tools.eq_(selector.forked_from(self.asset_type, obj2), obj)
+            if selector.can_fork(self.asset_type):
+                exit_code, outputs = self.call("fork", obj, obj2, prefix=tmp_prefix)
+                nose.tools.eq_(exit_code, 0, outputs)
+                selector.load()
+                s = self.storage_cls(tmp_prefix, obj2)
+                nose.tools.assert_true(s.exists())
+                nose.tools.eq_(selector.forked_from(self.asset_type, obj2), obj)
+            else:
+                exit_code, outputs = self.call("fork", obj, obj2, prefix=tmp_prefix)
+                nose.tools.assert_not_equal(exit_code, 0)
 
     def test_delete_local(self):
         obj = self.object_map["create"]