Tighten $currentOp and pinned cursor checks in kill_pinned_cursor.js

The test is assuming a find command is hitting a fail point for a pinned cursor by looking at the total open 'pinned' count in serverStatus. However, its possible that other background threads (e.g. PeriodicShardedIndexConsistencyChecker) hit the fail point and not the find command that the test is expecting. Instead of using serverStatus, the test should run a $currentOp and filter the output based on the 'failpointMsg' and the 'comment' string.