TST: Additions/updates to tests
wrote a test for https://github.com/pandas-dev/pandas/issues/56717

test function name is test_deprecation_warning_for_usage_of_aaply_map_index_method_of_styler_object
The original function that was tested was the Styler.applymap_index in the style module
The new test was appended to the test_style module file
The test checks for the FutureWarning

- [ ] closes #56717 
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).



Thanks @jomorodi 