on PPC/Debian Linux default stack size for a thread is too big.
As we use default thread settings in mysqltest, the
thread creation fails if we create lots of threads (as it
happens in flush.test). So now stack size is explicitly specified
for the mysqltest


client/mysqltest.c:
  Bug #28333 Test "flush" tries to create a new thread - on only one platform
  
  specify appropriate stack size for the 'query' thread