mixer test: Fix string handling
mixer test: Fix string handling

Some strings were not enforcing NUL termination.

mje-nz/PX4-Firmware,mcgill-robotics/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,dagar/Firmware,Aerotenna/Firmware,PX4/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,PX4/Firmware,PX4/Firmware,mje-nz/PX4-Firmware,krbeverx/Firmware,acfloria/Firmware,acfloria/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,PX4/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,Aerotenna/Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,dagar/Firmware,dagar/Firmware,jlecoeur/Firmware,Aerotenna/Firmware,dagar/Firmware,mje-nz/PX4-Firmware,mcgill-robotics/Firmware,jlecoeur/Firmware,acfloria/Firmware,acfloria/Firmware,krbeverx/Firmware,krbeverx/Firmware,mcgill-robotics/Firmware,acfloria/Firmware,mje-nz/PX4-Firmware,dagar/Firmware,jlecoeur/Firmware,krbeverx/Firmware,Aerotenna/Firmware,jlecoeur/Firmware,mcgill-robotics/Firmware,krbeverx/Firmware,PX4/Firmware,PX4/Firmware,krbeverx/Firmware,mje-nz/PX4-Firmware,PX4/Firmware,krbeverx/Firmware,jlecoeur/Firmware,acfloria/Firmware,acfloria/Firmware