t/lib-submodule-update: add test removing nested submodules

t/lib-submodule-update: add test removing nested submodules

The previous commit fixed a bug with the (no submodule) -> (nested
submodules) transition for commands in the unpack-trees machinery.

Let's add a test for the reverse transition (going from nested
submodules to no submodule), as it is not being tested currently.

While at it, uniformize the capitalization in the list of tests.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>