Correct instability in the rocksdb.drop_cf_* tests

CURRENT_TEST: rocksdb.drop_cf_during_manual_compaction
mysqltest: At line 51: Query 'create table create_cf ( id int, primary key (id) comment 'cfname=cf_primary_key' ) engine=rocksdb' failed.
ERROR 8018 (HY000): Column family ('cf_primary_key') is being dropped.

The result from queries just before the failure was:
set @old_debug = @@global.debug;
create table create_cf ( id int, primary key (id) comment 'cfname=cf_primary_key' ) engine=rocksdb;
drop table create_cf;
select * from information_schema.rocksdb_cfstats where cf_name='cf_primary_key';
CF_NAME	STAT_TYPE	VALUE
cf_primary_key	NUM_IMMUTABLE_MEM_TABLE	0
cf_primary_key	MEM_TABLE_FLUSH_PENDING	0
cf_primary_key	COMPACTION_PENDING	0
cf_primary_key	CUR_SIZE_ACTIVE_MEM_TABLE	728
cf_primary_key	CUR_SIZE_ALL_MEM_TABLES	728
cf_primary_key	NUM_ENTRIES_ACTIVE_MEM_TABLE	0
cf_primary_key	NUM_ENTRIES_IMM_MEM_TABLES	0
cf_primary_key	NON_BLOCK_CACHE_SST_MEM_USAGE	0
cf_primary_key	NUM_LIVE_VERSIONS	1
set @@global.debug = '+d,rocksdb_compact_column_family';
set @@global.debug = '+d,rocksdb_drop_cf';
set @@global.rocksdb_compact_cf = 'cf_primary_key';
set debug_sync = "now wait_for ready_to_mark_cf_dropped_in_compact_column_family";
select * from information_schema.rocksdb_cfstats where cf_name='cf_primary_key';
CF_NAME	STAT_TYPE	VALUE
cf_primary_key	NUM_IMMUTABLE_MEM_TABLE	0
cf_primary_key	MEM_TABLE_FLUSH_PENDING	0
cf_primary_key	COMPACTION_PENDING	0
cf_primary_key	CUR_SIZE_ACTIVE_MEM_TABLE	728
cf_primary_key	CUR_SIZE_ALL_MEM_TABLES	728
cf_primary_key	NUM_ENTRIES_ACTIVE_MEM_TABLE	0
cf_primary_key	NUM_ENTRIES_IMM_MEM_TABLES	0
cf_primary_key	NON_BLOCK_CACHE_SST_MEM_USAGE	0
cf_primary_key	NUM_LIVE_VERSIONS	1
set @@global.rocksdb_delete_cf = 'cf_primary_key';
select * from information_schema.rocksdb_cfstats where cf_name='cf_primary_key';
CF_NAME	STAT_TYPE	VALUE
cf_primary_key	NUM_IMMUTABLE_MEM_TABLE	0
cf_primary_key	MEM_TABLE_FLUSH_PENDING	0
cf_primary_key	COMPACTION_PENDING	0
cf_primary_key	CUR_SIZE_ACTIVE_MEM_TABLE	728
cf_primary_key	CUR_SIZE_ALL_MEM_TABLES	728
cf_primary_key	NUM_ENTRIES_ACTIVE_MEM_TABLE	0
cf_primary_key	NUM_ENTRIES_IMM_MEM_TABLES	0
cf_primary_key	NON_BLOCK_CACHE_SST_MEM_USAGE	0
cf_primary_key	NUM_LIVE_VERSIONS	1
set debug_sync = "now signal mark_cf_dropped_done_in_compact_column_family";
set @@global.debug = '+d,rocksdb_drop_cf';
select * from information_schema.rocksdb_cfstats where cf_name = 'cf_primary_key' ;
CF_NAME	STAT_TYPE	VALUE
cf_primary_key	NUM_IMMUTABLE_MEM_TABLE	0
cf_primary_key	MEM_TABLE_FLUSH_PENDING	0
cf_primary_key	COMPACTION_PENDING	0
cf_primary_key	CUR_SIZE_ACTIVE_MEM_TABLE	728
cf_primary_key	CUR_SIZE_ALL_MEM_TABLES	728
cf_primary_key	NUM_ENTRIES_ACTIVE_MEM_TABLE	0
cf_primary_key	NUM_ENTRIES_IMM_MEM_TABLES	0
cf_primary_key	NON_BLOCK_CACHE_SST_MEM_USAGE	0
cf_primary_key	NUM_LIVE_VERSIONS	1
set debug_sync = "now signal ready_to_drop_cf";
set debug_sync = "now wait_for drop_cf_done";
Timeout in wait_condition.inc for select count(*) = 0 from information_schema.rocksdb_cfstats where cf_name = 'cf_primary_key'
set @@global.debug = @old_debug;
safe_process[23784]: Child process: 23785, exit: 1