AuTest: Reuse venv if it exists already (#7178)
AuTest: Reuse venv if it exists already (#7178)

Before this change, every call to autest.sh would re-install the pipenv
virtual environment. This reuses the virtual environment on subsequent
runs to autest.sh.
duke8253/trafficserver,vmamidi/trafficserver,bryancall/trafficserver,SolidWallOfCode/trafficserver,SolidWallOfCode/trafficserver,duke8253/trafficserver,duke8253/trafficserver,bryancall/trafficserver,vmamidi/trafficserver,duke8253/trafficserver,SolidWallOfCode/trafficserver,vmamidi/trafficserver,pbchou/trafficserver,bryancall/trafficserver,bryancall/trafficserver,duke8253/trafficserver,SolidWallOfCode/trafficserver,pbchou/trafficserver,pbchou/trafficserver,SolidWallOfCode/trafficserver,bryancall/trafficserver,pbchou/trafficserver,pbchou/trafficserver,duke8253/trafficserver,bryancall/trafficserver,vmamidi/trafficserver,SolidWallOfCode/trafficserver,vmamidi/trafficserver,SolidWallOfCode/trafficserver,vmamidi/trafficserver,duke8253/trafficserver,pbchou/trafficserver