fix(installer): fail closed on test enumeration errors

tools/verified-installer-fetch.test.sh

@@ -1,6 +1,8 @@
 #!/usr/bin/env bash
 set -euo pipefail
 ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+# shellcheck source=tools/test-enumeration-assertions.sh
+source "$ROOT/tools/test-enumeration-assertions.sh"
 TMP="$(mktemp -d "${TMPDIR:-/tmp}/mosaic-fetch-contract.XXXXXX")"
 trap 'rm -rf "$TMP"' EXIT
 FAKE_BIN="$TMP/bin"; mkdir -p "$FAKE_BIN"
@@ -41,7 +43,7 @@ empty_sha="$(printf '' | sha256sum | awk '{print $1}')"
 mkdir -p "$TMP/downloads"
 output="$(TMPDIR="$TMP/downloads" PATH="$FAKE_BIN:$PATH" bash "$ROOT/tools/verified-installer-fetch.sh" fixture://ok "$ok_sha" -- marker)"
 [[ "$output" == 'executed:marker' ]]
-[[ -z "$(find "$TMP/downloads" -mindepth 1 -print -quit)" ]]
+test_assert_find_empty 'verified-installer temporary downloads' "$TMP/downloads" -mindepth 1
 printf '[test] PASS: digest-pinned fetched artifact executes and its temporary body is removed\n'
 
 for row in 'fixture://empty empty-body' 'fixture://failed failed-fetch'; do