Add sanity check to test_cache_read_from
Add sanity check to test_cache_read_from

Add sanity check per Mateo's comment here: https://rbcommons.com/s/twitter/r/3284/

Testing Done:
CI passed: https://travis-ci.org/pantsbuild/pants/builds/100447955

Reviewed at https://rbcommons.com/s/twitter/r/3299/

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