tox: Report coverage to user

Run 'coverage report' after a successful coverage run. There's enough
coverage-related stuff here at this point to warrant its own section.

Signed-off-by: Stephen Finucane <stephen@that.guru>

tox.ini

@@ -8,7 +8,6 @@ passenv =
 description =
     py{27,34,35,36,py}: Run unit tests against {envname}.
     du{11,12,13,14}: Run unit tests with the given version of docutils.
-    coverage: Run code coverage checks.
 
 # TODO(stephenfin) Replace this with the 'extras' config option when tox 2.4 is
 # widely available, likely some time after the Ubuntu 18.04 release
@@ -22,7 +21,6 @@ deps =
     du14: docutils==0.14
 setenv =
     SPHINX_TEST_TEMPDIR = {envdir}/testbuild
-    coverage: PYTEST_ADDOPTS = --cov sphinx --cov-config {toxinidir}/setup.cfg
 commands=
     {envpython} -Wall tests/run.py --durations 25 {posargs}
 
@@ -41,6 +39,15 @@ deps =
 commands =
     pylint --rcfile utils/pylintrc sphinx
 
+[testenv:coverage]
+description =
+    Run code coverage checks.
+setenv =
+    PYTEST_ADDOPTS = --cov sphinx --cov-config {toxinidir}/setup.cfg
+commands =
+    {[testenv]commands}
+    coverage report
+
 [testenv:mypy]
 description =
     Run type checks.