Add --optimizer_trace option to mysqltest

This enables optimizer_trace output for the next SQL command.
Identical as if one would have done:
- Store value of @@optimizer_trace
- Set @optimizer_trace="enabled=on"
- Run query
- SELECT * from OPTIMIZER_TRACE
- Restore value of @@optimizer_trace

This is a great time saver when one wants to quickly check the optimizer
trace for a query in a mtr test.

mysql-test/main/opt_trace.test

@@ -48,12 +48,12 @@ create view v1 as select a from t1 group by b;
 create view v2 as select a from t2;
 
 --echo # Mergeable view
+--optimizer_trace
 explain select * from v2 ;
-select * from information_schema.OPTIMIZER_TRACE;
 
 --echo # Non-Mergeable view
+--optimizer_trace
 explain select * from v1 ;
-select * from information_schema.OPTIMIZER_TRACE;
 drop table t1,t2;
 drop view v1,v2;