Bug #58087 mysqltest re-evaluates 'let' expressions infinitely

Results from query is sent for evaluation
Break recursion by asking for ` to be ignored

mysql-test/t/mysqltest.test

@@ -854,6 +854,13 @@ let $var2= `failing query`;
 echo $var2;
 EOF
 
+create table t1 (a varchar(100));
+insert into t1 values ('`select 42`');
+let $a= `select * from t1`;
+# This should output `select 42`, not evaluate it again to 42
+echo $a;
+drop table t1;
+
 --error 1
 --exec $MYSQL_TEST < $MYSQLTEST_VARDIR/tmp/let.sql 2>&1