Fix `test_add_column_with_timestamp_type` failure

This test failed due to dirty schema cache. It is needed to call
`clear_cache!` when using same named table with different definition.

https://travis-ci.org/rails/rails/jobs/310627767#L769-L772

activerecord/test/cases/migration/change_schema_test.rb

@@ -264,19 +264,18 @@ def test_add_column_with_timestamp_type
           t.column :foo, :timestamp
         end
 
-        klass = Class.new(ActiveRecord::Base)
-        klass.table_name = "testings"
+        column = connection.columns(:testings).find { |c| c.name == "foo" }
 
-        assert_equal :datetime, klass.columns_hash["foo"].type
+        assert_equal :datetime, column.type
 
         if current_adapter?(:PostgreSQLAdapter)
-          assert_equal "timestamp without time zone", klass.columns_hash["foo"].sql_type
+          assert_equal "timestamp without time zone", column.sql_type
         elsif current_adapter?(:Mysql2Adapter)
-          assert_equal "timestamp", klass.columns_hash["foo"].sql_type
+          assert_equal "timestamp", column.sql_type
         elsif current_adapter?(:OracleAdapter)
-          assert_equal "TIMESTAMP(6)", klass.columns_hash["foo"].sql_type
+          assert_equal "TIMESTAMP(6)", column.sql_type
         else
-          assert_equal klass.connection.type_to_sql("datetime"), klass.columns_hash["foo"].sql_type
+          assert_equal klass.connection.type_to_sql("datetime"), column.sql_type
         end
       end
 

activerecord/test/cases/migration/compatibility_test.rb

@@ -141,6 +141,8 @@ def migrate(x)
           Testing.create!
           ActiveRecord::Migrator.new(:up, [migration]).migrate
           assert_equal ["foobar"], Testing.all.map(&:foo)
+        ensure
+          ActiveRecord::Base.clear_cache!
         end
       end
     end