Restore missing --test-path to [coverage] tox env

Remove outdated comment.

Remove redundant usedevelop.

[skip ci]

tox.ini

@@ -17,7 +17,6 @@ commands =
     zope-testrunner -u --test-path=src []
 # Only run functional tests if unit tests pass.
     zope-testrunner -f --test-path=src []
-# without explicit deps, setup.py test will download a bunch of eggs into $PWD
 deps =
     .[test]
 
@@ -32,10 +31,9 @@ deps = {[testenv]deps}
 
 [testenv:coverage]
 basepython = python2.7
-usedevelop = true
 commands =
-    coverage run --source=ZODB -m zope.testrunner []
+    coverage run --source=ZODB -m zope.testrunner --test-path=src []
     coverage report
 deps =
-    coverage
     {[testenv]deps}
+    coverage