Make the main py3 tox environment an alias/merge of other test environments

Having tox call tox is a bit weird, but the alternatives are:
 - duplication of the test command, which is a bit sad
 - replacing all the positive factors with conjunction of negative factors, e.g.
   `core` -> `!db-!server`, which makes the logic painful to follow

One day, tox will have environment aliases which will make this horror moot :-)

(Idea pulled from a comment in tox-dev/tox#427)

Author: olasd

tox.ini

@@ -8,25 +8,21 @@ deps =
   db: .[db]
   db: pifpaf
   server: .[http]
+  cover: pytest-cov
 commands =
   db: pifpaf run postgresql -- \
     pytest --doctest-modules \
+  slow: --hypothesis-profile=slow \
+  cover: --cov={envsitepackagesdir}/swh/core --cov-branch \
   core: {envsitepackagesdir}/swh/core/tests \
   db: {envsitepackagesdir}/swh/core/db/tests \
   server: {envsitepackagesdir}/swh/core/api/tests \
     {posargs}
 
 [testenv:py3]
-deps =
-  .[testing]
-  pytest-cov
-  pifpaf
-commands =
-  pifpaf run postgresql -- \
-    pytest --doctest-modules \
-	       --hypothesis-profile=slow \
-	       --cov={envsitepackagesdir}/swh/core --cov-branch \
-		   {envsitepackagesdir}/swh/core {posargs}
+skip_install = true
+deps = tox
+commands = tox -e py3-core-db-server-slow-cover -- {posargs}
 
 [testenv:flake8]
 skip_install = true