set pytest rootdir

With pytest 6.1.0 the root directory calculation logic has
changed and is unreliable on our jenkins setup.   make
sure this is set explicitly.

Change-Id: I1d48b887030c32d2f3361fc36e65e8a988d2ba81
References: pytest-dev/pytest#7807

tox.ini

@@ -15,7 +15,7 @@ install_command=python -m pip install {env:TOX_PIP_OPTS:} {opts} {packages}
 usedevelop=
      cov: True
 
-deps=pytest<6.1
+deps=pytest>4.6  # this can be 6.x once we are on python 3 only
      pytest-xdist
      greenlet
      mock; python_version < '3.3'
@@ -57,7 +57,7 @@ setenv=
     PYTHONNOUSERSITE=1
     MEMUSAGE=--nomemory
     SQLALCHEMY_WARN_20=true
-    BASECOMMAND=python -m pytest --log-info=sqlalchemy.testing
+    BASECOMMAND=python -m pytest --rootdir {toxinidir} --log-info=sqlalchemy.testing
 
     WORKERS={env:TOX_WORKERS:-n4  --max-worker-restart=5}