Tests: clean up submodule recursive helpers

This continues the work in commit d3b5a49 ("Tests: clean up and document
submodule helpers", 2017-11-08).

Factor out the commonalities from
test_submodule_switch_recursing_with_args() and
test_submodule_forced_switch_recursing_with_args() in
lib-submodule-update.sh, and document their usage. Some tests differ
slightly in their test assertions; I have used the superset of those
assertions in that case.

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

Author: jonathantanmy