Merge branch 'fc/show-non-empty-errors-in-test'

* fc/show-non-empty-errors-in-test:
test: test_must_be_empty helper

diff --cc t/t3400-rebase.sh
Simple merge

diff --cc t/t3903-stash.sh
Simple merge

diff --cc t/t7400-submodule-basic.sh
Simple merge

diff --cc t/test-lib-functions.sh
Simple merge