Makefile: use px4_sitl_test board for `make tests`
Makefile: use px4_sitl_test board for `make tests`

This means that we can use the normal host scheduling (not lockstep) for
the unit tests.

In the future we might add unit tests using lockstep as well.

PX4/Firmware,dagar/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,krbeverx/Firmware,PX4/Firmware,krbeverx/Firmware,krbeverx/Firmware,PX4/Firmware,dagar/Firmware,krbeverx/Firmware,PX4/Firmware,acfloria/Firmware,PX4/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,acfloria/Firmware,krbeverx/Firmware,dagar/Firmware,dagar/Firmware,dagar/Firmware,mje-nz/PX4-Firmware