Fix failing functional test for coverage plugin

Coverage plugin has extra requirements (cov-core>=1.12; see setup.py) which are not installed for tox and CI tests. 

This pull request:
1. makes requirements for test envs match those in setup.py;
2. fixes the test to also work on Windows.

PS. The failure in question is:
```
FAIL: test_run (nose2.tests.functional.test_coverage.TestCoverage)

Traceback (most recent call last):
File "nose2/tests/functional/test_coverage.py", line 13, in test_run
self.assertTestRunOutputMatches(proc, stderr='lib/mod1 8 5 38%')
File "nose2/tests/_common.py", line 63, in assertTestRunOutputMatches
testf(util.safe_decode(cmd_stderr), stderr)
AssertionError: Regexp didn't match: 'lib/mod1 8 5 38%' not found in u'test1 (test_coverage.TestLib) ... ok\n\n----------------------------------------------------------------------\nRan 1 test in 0.001s\n\nOK\n'
```