Add `_compare()` function to test_helper.bash.

test/test_helper.bash

@@ -24,3 +24,22 @@ teardown() {
     rm "${HOSTS_PATH}"
   fi
 }
+
+###############################################################################
+# Helpers
+###############################################################################
+
+# _compare()
+#
+# Usage:
+#   _compare <expected> <actual>
+#
+# Descriotion:
+#   Compare the content of a variable against an expected value. When used
+#   within a `@test` block the output is only displayed when the test fails.
+_compare() {
+  local _expected="${1:-}"
+  local _actual="${2:-}"
+  printf "expected:\n%s\n" "${_expected}"
+  printf "actual:\n%s\n" "${_actual}"
+}