test-fixes for 32-bit

mysql-test/t/variables.test:
  Test for 10351 ("Max_Heap_Table_Size cannot be set to 4 gb")
  now throws a warning on 32-bit, as it should. As we're not
  actually interested in the warning but only in whether the
  variable is set to a correct value afterwards, we disable
  warnings for the duration of that test case so we won't have
  to split the test into a 32-bit and a 64-bit flavour for no
  real reason.