west: build: allow --test-item to accept twister style instance names

This will allow something like this:

west build -b qemu_x86  -T samples/hello_world/sample.basic.helloworld

instead of:

cd samples/hello_world
west build -b qemu_x86  -T sample.basic.helloworld

Using this option will allow to take twister output and just pass it to
west to reproduce failures.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>