Tests: clean up and document submodule helpers

Tests: clean up and document submodule helpers

Factor out the commonalities from test_submodule_switch() and
test_submodule_forced_switch() in lib-submodule-update.sh, and document
their usage.

This also makes explicit (through the KNOWN_FAILURE_FORCED_SWITCH_TESTS
variable) the fact that, currently, all functionality tested using
test_submodule_forced_switch() do not correctly handle the situation in
which a submodule is replaced with an ordinary directory.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>