Update test_describe.py
Added test case

Modified the .describe() method to avoid adding the 50th percentile by default when a custom percentiles list is provided.
Added a test case to verify the behavior when a single percentile is passed.
@mdjaved24 Hi, I noticed you already open a PR (https://github.com/pandas-dev/pandas/pull/60986) related to same issue, you should commit and push to same PR instead of creating two separate PRs. Agreed. Please continue working on this feature in https://github.com/pandas-dev/pandas/pull/60986 so closing this one out