samples/drivers/mbox: Fix test regex

In 9d7ee123
the regex used to verify the sample is doing what it should was changed
in a way in which it cannot possibly pass.
Let's fix it.

While at it, in the same commit, the simulator version of the test was
merged with the real_hw version, but the test name kept "real_hw" which
is now missleading => let's fix it.

And let's set as integration platform the simulated target, so it
can actually be runtime tested in CI.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>