Followup of rails#27596, test output of "run" method as well

- rails#27596 Updated "run" and "migrate" method for returning correct
   value but only added test for "migrate" method. This commit adds a
   test for "run" as well.

activerecord/test/cases/migrator_test.rb

@@ -290,7 +290,7 @@ def test_migrator_going_down_due_to_version_target
     assert_equal [[:up, 1], [:up, 2], [:up, 3]], calls
   end
 
-  def test_migrator_output
+  def test_migrator_output_when_running_multiple_migrations
     _, migrator = migrator_class(3)
 
     result = migrator.migrate("valid")
@@ -304,6 +304,13 @@ def test_migrator_output
     assert_equal(1, result.count)
   end
 
+  def test_migrator_output_when_running_single_migration
+    _, migrator = migrator_class(1)
+    result = migrator.run(:up, "valid", 1)
+
+    assert_equal(1, result.version)
+  end
+
   def test_migrator_rollback
     _, migrator = migrator_class(3)