tests: Remove a pmemberᵒ test

The pmemberᵒ function was changed during the course of chapter three of
The Reasoned Schemer, the test in frame 3.80 does not apply to the final
version of the function. It is no point keeping it there, disabled, when
it became obsolete. For that reason, it is now removed.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>