MDEV-20487: Fix a test

The test rpl.rpl_failed_drop_tbl_binlog exercises a scenario where
the adaptive hash index is enabled. Try to explicitly enable the
adaptive hash index for this test, and skip the test if it does not
succeed (that is, if the server was not built WITH_INNODB_AHI=ON).

mysql-test/suite/rpl/t/rpl_failed_drop_tbl_binlog.opt

+--loose-innodb-adaptive-hash-index

mysql-test/suite/rpl/t/rpl_failed_drop_tbl_binlog.test

@@ -26,6 +26,11 @@
 --source include/have_binlog_format_statement.inc
 --source include/master-slave.inc
 
+if (!`select @@GLOBAL.innodb_adaptive_hash_index`)
+{
+  --skip Need innodb_adaptive_hash_index
+}
+
 create table t1 (a int) engine=innodb;
 create table t2 (b longblob) engine=innodb;
 create table t3 (c int) engine=innodb;