atest/run.py: Allow running tests also when not in project root.
atest/run.py: Allow running tests also when not in project root.

The bug only occurred if `invoke` was installed. Now the requirement
to be in the project root only affects situation where the jar
distribution is created.

HelioGuilherme66/robotframework,HelioGuilherme66/robotframework,robotframework/robotframework,robotframework/robotframework,HelioGuilherme66/robotframework,robotframework/robotframework