test/kernel/mbox: Drop needless _1cpu_ from test cases

This test is already running with CONFIG_MP_NUM_CPUS=1.  All those
1cpu declarations are needless.  And some of them (like the init
tests) have side effects that make it difficult to do things like
"filter for only MP cases".

(Indeed, this is a heavily MP-unsafe test; almost all cases written to
rely on ordering between a parent thread and its child.  And that's
doubly so for COHERENE platforms because lots and lots of the test
objects are on stacks.  MP_NUM_CPUS=1 is definitely the right thing
here.)

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>