tests: mem_protect/syscalls: multiple buffers for stress test

This modifies string_copy() to have an addition ID as function
argument so that it can copy into different buffers during
the multi-threaded stress test, instead of all threads writing
into the same buffer. This is to prevent a situation where
one thread writes incorrect data but being overwritten by
another thread with correct data, thus making the following
strcmp() passing for both threads. Logically this should never
happen since the same function is being called which should
give same result, but it is simply to test for weird
situations.

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