fixup! submodules: Add the lib-submodule-update.sh test library

- call test_git_directory_is_unchanged before test_submodule_content
fixup! rebase: add t3426 for submodule updates

- use expect instead of expected
fixup! revert: add t3513 for submodule updates

- use expect instead of expected
fixup! stash: add t3906 for submodule updates

- use expect instead of expected
fixup! bisect: add t6041 for submodule updates

- use expect instead of expected