tests: kernel: mem_protect: Adjust priv exec tests

This patch removes the extraneous priv_insn test as it is a duplicate
of the following test that writes to the control register.  For ARM,
unprivileged contexts which access control registers does not result
in a fault.  It results in no modification of the register, so we have
to check that a modification occurred.

Signed-off-by: Andy Gross <andy.gross@linaro.org>