Moved the test case for the bug into a separate file (and restored the 
original innodb_mysql test setup).
Used the new wait_show_condition test macro to avoid the usage of sleep

mysql-test/include/wait_show_condition.inc:
  Bug #42419: new test macro to wait for a row in SHOW to have a certain value.
mysql-test/r/innodb_bug42419.result:
  Bug #42419: test case
mysql-test/r/innodb_mysql.result:
  Bug #42419: revert to the original innodb_mysql test
mysql-test/t/innodb_bug42419.test:
  Bug #42419: test case
mysql-test/t/innodb_mysql-master.opt:
  Bug #42419: revert to the original innodb_mysql test
mysql-test/t/innodb_mysql.test:
  Bug #42419: revert to the original innodb_mysql test