tox.ini: Use tests installed files instead of working directory

tox.ini

@@ -6,7 +6,7 @@ deps =
   -rrequirements-test.txt
   .
 commands =
-  pytest --doctest-modules swh/core/tests {posargs}
+  pytest --doctest-modules {envsitepackagesdir}/swh/core/tests {posargs}
 
 [testenv:py3-db]
 deps =
@@ -15,14 +15,14 @@ deps =
   pifpaf
 commands =
   pifpaf run postgresql -- \
-    pytest swh/core/db/tests {posargs}
+    pytest {envsitepackagesdir}/swh/core/db/tests {posargs}
 
 [testenv:py3-server]
 deps =
   -rrequirements-test.txt
   .[http]
 commands =
-  pytest swh/core/api/tests {posargs}
+  pytest {envsitepackagesdir}/swh/core/api/tests {posargs}
 
 [testenv:py3]
 deps =
@@ -34,7 +34,7 @@ commands =
     pytest --doctest-modules \
 	       --hypothesis-profile=slow \
 	       --cov=swh --cov-branch \
-		   {posargs}
+		   {envsitepackagesdir} {posargs}
 
 [testenv:flake8]
 skip_install = true