mpu_stack_guard_test: add more thread for test

This test launches several threads. Only last thread will overflow the
stack and an mpu exception will occur. The other threads are regulary
suspended, this triggers access to kthread structure. This tests is
failed on st and nxp platform, if 32 bytes alignement is not set.

Signed-off-by: Michel Jaouen <michel.jaouen@st.com>