lib-submodule-update.sh: define tests for recursing into submodules

lib-submodule-update.sh: define tests for recursing into submodules

Currently lib-submodule-update.sh provides 2 functions
test_submodule_switch and test_submodule_forced_switch that are used by a
variety of tests to ensure that submodules behave as expected. The current
expected behavior is that submodules are not touched at all (see
42639d2317a for the exact setup).

In the future we want to teach all these commands to recurse
into submodules. To do that, we'll add two testing functions to
submodule-update-lib.sh: test_submodule_switch_recursing and
test_submodule_forced_switch_recursing.

These two functions behave in analogy to the already existing functions
just with a different expectation on submodule behavior. The submodule
in the working tree is expected to be updated to the recorded submodule
version. The behavior is analogous to e.g. the behavior of files in a
nested directory in the working tree, where a change to the working tree
handles any arising directory/file conflicts just fine.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>