test: stop spawned mini when readiness times out

A slow-starting server otherwise survives the failure path and keeps
the S3 port occupied for the next run.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

test/s3/error_compat/Makefile

@@ -71,6 +71,7 @@ start-server: check-deps
 	echo "SeaweedFS S3 server failed to start"; \
 	echo "=== Server logs ==="; \
 	test -f weed-test.log && cat weed-test.log || true; \
+	$(MAKE) --no-print-directory stop-server; \
 	exit 1
 
 stop-server: