TEST: Add tests for lower() and lcase() function

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Add tests for `lower()` and `lcase()` functions once #1192 is merged.
Details of the above functions can be referred from https://github.com/4paradigm/OpenMLDB/issues/784

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Look at the merged PR #1192 to understand the implementation of lower() and lcase() functions. Refer to issue #784 for function details. Write tests in the project's testing framework, likely in a test file for SQL functions. Verify the functions work correctly with various string inputs.

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