twister/pytest: Inform pytest which platform a test is being done for

While the option to inform pytest about which platform a test is for
(platform being the board, such as mps2/an385) is available, it wasn't
being used. This patch fixes that, by building a pytest command which
includes the platform.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>