Before this fix, the test myisam_file_io executed:
- (a) an update on setup_instrument to disable non myisam file io instruments
- (b) a truncate on events_waits_history_long
and later
- (c) a select on events_waits_history_long

Surprisingly, events that were supposed to be disabled in (a) and removed in (b)
still were found in (c).

This happened for events such as 
  wait/io/file/innodb/innodb_data_file fil0fil.c: sync  
because the sync was started before (a) and completed after (b),
and as a consequence was added in the performance schema history, as expected.

Presence of these records in the history made the test fail. 

This fix makes the test script more robust to account for extra spill waits records in (c).