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

iceberg-python/tests/catalog/test_sql.py

Lines 113 to 117 in 9857107

	props = {
	"uri": "sqlite:///:memory:",
	"warehouse": f"file://{warehouse}",
	}
	catalog = SqlCatalog(catalog_name, **props)

[hussein-awala]

@Fokko I agree with @kevinjqliu on the alias proposition, I implemented #1140 which makes InMemoryCatalog a subclass of SqlCatalog, and overrides some methods to simplify the tests.