tests/kernel/thread_stack: Increase heap size

This test was written to use a TINY system heap (64 bytes) from which
it has to allocate on behalf of a userspace process.  The change in
convention from mem_pool (where the byte count now includes metadata
overhead) means it runs out of space.  Bump to 192 bytes.  Still tiny.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
diff --git a/tests/kernel/threads/thread_stack/prj.conf b/tests/kernel/threads/thread_stack/prj.conf
index 65c85bb..b6b5951 100644
--- a/tests/kernel/threads/thread_stack/prj.conf
+++ b/tests/kernel/threads/thread_stack/prj.conf
@@ -3,4 +3,4 @@
 CONFIG_THREAD_STACK_INFO=y
 CONFIG_APPLICATION_DEFINED_SYSCALL=y
 CONFIG_TEST_USERSPACE=y
-CONFIG_HEAP_MEM_POOL_SIZE=64
+CONFIG_HEAP_MEM_POOL_SIZE=192