wait_server_start: warn if lsof is not available

If lsof is not available, wait_server_start uses a fixed timeout,
which can trigger a race condition if the timeout turns out to be too
short. Emit a warning so that we know this is going on from the test
logs.
diff --git a/tests/compat.sh b/tests/compat.sh
index 17153f1..dcbe1bf 100755
--- a/tests/compat.sh
+++ b/tests/compat.sh
@@ -780,6 +780,7 @@
         done
     }
 else
+    echo "Warning: lsof not available, wait_server_start = sleep"
     wait_server_start() {
         sleep 2
     }