Split up tests/db_engine_test.py (#8449)

* Split up db_engine_specs_test.py into a number of targeted files

* Remove db_engine_specs_test.py

* isort

tests/db_engine_specs/base_tests.py

@@ -0,0 +1,28 @@
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+from superset.db_engine_specs.mysql import MySQLEngineSpec
+from superset.models.core import Database
+from tests.base_tests import SupersetTestCase
+
+
+class DbEngineSpecTestCase(SupersetTestCase):
+    def sql_limit_regex(
+        self, sql, expected_sql, engine_spec_class=MySQLEngineSpec, limit=1000
+    ):
+        main = Database(database_name="test_database", sqlalchemy_uri="sqlite://")
+        limited = engine_spec_class.apply_limit_to_sql(sql, limit, main)
+        self.assertEqual(expected_sql, limited)