[Improvement] Backend storage repeatedly restarts in a UT class.

[mchades]

What would you like to be improved?

in the test class, each UT will restart H2, MySQL, and PostgreSQL, which is unnecessary but also causes tests to slow since they need to start and stop the container repeatedly.

https://github.com/apache/gravitino/blob/fb3d900ff0e70514f86dc8462ba07e5611bc4015/core/src/test/java/org/apache/gravitino/storage/TestEntityStorage.java#L116-L123

How should we improve?

reuse the backend storage instance in the class