According to buildbot cross-reference atomic.alter_table is failing ~10
times a day due to 900 seconds test case timeout. Split it into two
tests: atomic.alter_table_myisam and atomic.alter_table_innodb.
It should reduce failure frequency down to once a day or so, similarly
to atomic.alter_table_aria test.
Diabling InnoDB for MyISAM/Aria/RocksDB tests makes them 20-35% faster.
