ci/test: add tests for exact expected artifacts.

Cache reuse in zig is currently quite fragile (we are using a forked
version with github.com/ziglang/zig/issues/13051), so ensure that it
does not regress when zig SDK upgrades.

ci/test

@@ -1,2 +1,15 @@
-#!/bin/sh
-exec bazel test "$@" ...
+#!/usr/bin/env bash
+set -euo pipefail
+
+bazel test "$@" ...
+
+# /tmp/bazel-zig-cc should be empty for the test below to be valid.
+# This test ensures that github.com/ziglang/zig/issues/13050 does not
+# regress from a "very high"-level perspective.
+find /tmp/bazel-zig-cc -name mutex_destructor.o -execdir file {} \; | \
+    sort | uniq -c | sort -rn > /tmp/got_cache
+
+diff -u ci/testdata/want_cache /tmp/got_cache || {
+    >&2 echo "ERROR: unexpected artifacts."
+    >&2 echo "Was /tmp/bazel-zig-cc empty before the test?"
+}