fix assertions in test_graph to actually test results

[asottile]

I used this as an example in an upcoming video -- the current code doesn't properly validate the exception messages for two reasons (1) the | character allows it to unintentionally match a substring (for example raise ValueError("A cycle was detected") passes the test) and (2) the match argument uses re.search to find the substring match

personally I recommend not using match= at all and instead assert based on the excinfo object -- but this is the minimal "correct" change without annoying the PT011 linter