Remove `InMemoryCatalog` from the test-codebase

[Fokko]

Feature Request / Improvement

InMemoryCatalog can be replaced by the SqlCatalog with Sqlite as a backend. Less code is more :)

[kevinjqliu]

perhaps its a good idea to alias a new InMemoryCatalog implementation using SqlCatalog such as https://github.com/apache/iceberg-python/blob/9857107561d2267813b7ce150b01b4e6ac4b3e34/tests/catalog/test_sql.py#L113-L117

[hussein-awala]

@Fokko I agree with @kevinjqliu on the alias proposition, I implemented https://github.com/apache/iceberg-python/pull/1140 which makes InMemoryCatalog a subclass of SqlCatalog, and overrides some methods to simplify the tests.