beam: CHECK resolves root from \$0 instead of hardcoded path

wizard-work/beam/CHECK

@@ -1,6 +1,6 @@
 #!/bin/bash
 # CHECK: verify all beam reliability primitives
 set -e
-cd /root/troupe-standup/wizard-work/beam
+cd "$(dirname "$(readlink -f "$0")")"
 python3 -m unittest discover -p 'test_*.py' -v
 python3 toposort/test_toposort.py