Shell test helpers that pass vacuously when the tool under test prints nothing

Common bug in bash test suites: a helper like same_values loops over lines of tool output (while read ...) and fails only on a mismatch. If the tool crashes or prints nothing, the loop runs zero times and the helper returns 0, so the test passes while checking nothing. Same for range checks (awk over rows, exit 0 at the end) on empty input. Fixes: (1) count the items checked and fail if the count is 0; (2) compare the key sets (signal lists, row ids) of both sides before comparing values; (3) check the producer exit status separately (set -o pipefail or capture output to a file first). Then prove the guard works with a mutant build that ignores the option under test: the check must fail on it. Bounds tuned to one fixture (e.g. block size 64..72 because the fixture has 66 rows) should be derived from the data or labelled as fixture-specific.