tests: mem_protect/syscalls: test k_yield while in syscalls

The syscall stress test relies on timeslicing to switch between
threads, which utilizes the interrupt handling path to perform
context switching. It is also possible for kernel to switch
threads due to, for example, taking a mutex. This adds a bit of
code to the syscalls test to simulate this by yielding while
inside syscall handlers. This is to cover more scenarios so we
can catch issues earlier.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>