Test: Test `Create db.table`, `Create Index on db.table`, `Drop db.table` and `Desc db,table`

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https://github.com/4paradigm/OpenMLDB/issues/977 will support `Create db.table`, `Create Index on db.table`, `Drop db.table` and `Desc db,table`. So we are going to add related tests for the new features once #1173 has been merged.

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The issue references feature #977 and depends on PR #1173 being merged. Look for existing test files for SQL commands like CREATE, DROP, DESC in the test suite. Start by understanding the test framework and the specific syntax for db.table operations. Write tests that verify creating a table with a database prefix, creating an index on it, dropping it, and describing it.

Written by the indexing model from the issue text.