chore: import upstream snapshot with attribution

examples/sandbox/docs/repo/tests/test_credit_note.sh

@@ -0,0 +1,16 @@
+#!/bin/sh
+set -eu
+
+actual_positive="$(sh credit_note.sh Northwind 12.50)"
+if [ "$actual_positive" != 'Credit note for Northwind: $12.50 credit.' ]; then
+    printf 'expected positive case to pass, got: %s\n' "$actual_positive" >&2
+    exit 1
+fi
+
+actual_negative="$(sh credit_note.sh Northwind -12.50)"
+if [ "$actual_negative" != 'Credit note for Northwind: $12.50 credit.' ]; then
+    printf 'expected negative case to pass, got: %s\n' "$actual_negative" >&2
+    exit 1
+fi
+
+printf '2 passed\n'