Fix main.stack on Windows

A part of the test, that tests that a frame of recursive SP takes the
same amount of stack, relies on the fact that 3 calls to that
SP are executed in the same OS thread. This is not guaranteed with
threadpool(not with Windows native threadpool, anyway)

Fix the test to execute SP calls in the same thread, as semicolon-separated
batched command.

mysql-test/main/stack.result

@@ -37,6 +37,7 @@ $$
 call recursion(0,2,@s1);
 call recursion(0,3,@s2);
 call recursion(0,4,@s3);
+$$
 select @s1 > 0 && @s2 > 0 && @s3 > 0;
 @s1 > 0 && @s2 > 0 && @s3 > 0
 1