dist/pythinlibs/testrunner: dynamic changing of test timeout
dist/pythinlibs/testrunner: dynamic changing of test timeout

Few flashers require a longer time to flash code and reset the device and
requires the test script to wait for a longer time before timing out.
This commit adds a environment variable "RIOT_TEST_TIMEOUT"  that can be
set by the user to vary the timeout in accordance to the requirements
of the system

on-behalf-of: @sparkmeter <263f9e8de33e9841989411f368ce9eb964cc9d22@sparkmeter.io>

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