Make 'tox -e coverage' work

It didn't work for me, because nose would crawl inside manual_tests, try
to run test_fstest.py, and fail (because that test is manual and
requires extra setup, like putting fstest.py on PYTHONPATH).

I also don't think that we want to exclude test layers from coverage
measurements (nose doesn't support them).

This change makes 'tox -e coverage' use zope.testrunner like it does for
regular tests.

.coveragerc

+[run]
+# source = ...
+# omit = ...
+# cover_pylib = False
+# branch = True
+
+[report]
+exclude_lines =
+    pragma: nocover
+    if __name__ == ['"]__main__['"]:
+    assert False

tox.ini

@@ -30,26 +30,11 @@ commands =
 deps = {[testenv]deps}
 
 [testenv:coverage]
-basepython =
-    python2.7
+basepython = python2.7
+usedevelop = true
 commands =
-#   The installed version messes up nose's test discovery / coverage reporting
-#   So, we uninstall that from the environment, and then install the editable
-#   version, before running nosetests.
-    pip uninstall -y ZODB
-    pip install -e .
-    nosetests --with-xunit --with-xcoverage
+    coverage run --source=ZODB -m zope.testrunner --test-path=src --auto-color --auto-progress
+    coverage report
 deps =
-    nose
     coverage
-    nosexcover
-    BTrees
-    ZConfig
-    manuel
-    persistent
-    six
-    transaction
-    zc.lockfile
-    zdaemon
-    zope.interface
-    zope.testing
+    {[testenv]deps}