[3.13] Docs: fix spelling of "test case" in `unittest` documentation (GH-134137) (#134139)

Docs: fix spelling of "test case" in `unittest` documentation (GH-134137)
(cherry picked from commit b41d79c776af4bfa5d28fc2036c9137978294b6a)

Co-authored-by: Clifford Gama <53076065+cliff688@users.noreply.github.com>