Add "[test]" extra depends (and include in [devel])

We have them duplicated in tox.ini.  @jwodder prefers to describe
them explicitly

Author: yarikoptic

setup.cfg

@@ -47,8 +47,13 @@ install_requires =
 [options.extras_require]
 benchmarks =
     asv[virtualenv] ~= 0.6.0, < 0.6.2
+test =
+    pytest-cov
+    pytest-mock
+    pytest-rerunfailures
 devel =
     %(benchmarks)s
+    %(test)s
     pre-commit
 all =
     %(devel)s