Test for `TransitionGroup`

Editor assessment

test(TransitionGroup) states that TransitionGroup is tested, with a pull request reference in the header. The scope and type are accurate, the subject is imperative, and there is no trailing period. Slight redundancy appears because the type and subject both say test, and the message offers no description of the covered behavior or the bug being guarded against. For a test addition that is a minor omission, and the message remains clear, correct, and publishable.