tests/rng: expect shell prompt in test
tests/rng: expect shell prompt in test

Be more pedantic in expected output for shell commands.

For slow boards, `ws430-v1_3b/arduino-mega2560/msba2`, some commands
were sent before the output of the previous command.

kaspar030/RIOT,miri64/RIOT,rfuentess/RIOT,toonst/RIOT,toonst/RIOT,cladmi/RIOT,RIOT-OS/RIOT,miri64/RIOT,kYc0o/RIOT,josephnoir/RIOT,OlegHahm/RIOT,smlng/RIOT,x3ro/RIOT,mfrey/RIOT,josephnoir/RIOT,RIOT-OS/RIOT,toonst/RIOT,kYc0o/RIOT,OTAkeys/RIOT,miri64/RIOT,OTAkeys/RIOT,miri64/RIOT,ant9000/RIOT,jasonatran/RIOT,cladmi/RIOT,yogo1212/RIOT,rfuentess/RIOT,yogo1212/RIOT,miri64/RIOT,smlng/RIOT,kaspar030/RIOT,jasonatran/RIOT,aeneby/RIOT,basilfx/RIOT,kaspar030/RIOT,x3ro/RIOT,mtausig/RIOT,aeneby/RIOT,kYc0o/RIOT,OTAkeys/RIOT,cladmi/RIOT,cladmi/RIOT,mtausig/RIOT,RIOT-OS/RIOT,basilfx/RIOT,rfuentess/RIOT,josephnoir/RIOT,x3ro/RIOT,smlng/RIOT,mfrey/RIOT,RIOT-OS/RIOT,mtausig/RIOT,aeneby/RIOT,authmillenon/RIOT,toonst/RIOT,OlegHahm/RIOT,cladmi/RIOT,OlegHahm/RIOT,ant9000/RIOT,yogo1212/RIOT,mtausig/RIOT,basilfx/RIOT,yogo1212/RIOT,toonst/RIOT,authmillenon/RIOT,jasonatran/RIOT,authmillenon/RIOT,smlng/RIOT,authmillenon/RIOT,rfuentess/RIOT,smlng/RIOT,kaspar030/RIOT,authmillenon/RIOT,mfrey/RIOT,RIOT-OS/RIOT,aeneby/RIOT,mtausig/RIOT,aeneby/RIOT,kYc0o/RIOT,kaspar030/RIOT,basilfx/RIOT,OlegHahm/RIOT,authmillenon/RIOT,x3ro/RIOT,jasonatran/RIOT,ant9000/RIOT,OTAkeys/RIOT,mfrey/RIOT,OTAkeys/RIOT,kYc0o/RIOT,ant9000/RIOT,yogo1212/RIOT,yogo1212/RIOT,mfrey/RIOT,x3ro/RIOT,basilfx/RIOT,ant9000/RIOT,OlegHahm/RIOT,josephnoir/RIOT,jasonatran/RIOT,josephnoir/RIOT,rfuentess/RIOT