Use a relative cache_dir in test because of how arguments are parsed on Windows

We use shlex to parse command-line arguments and PYTEST_ADDOPTS, so passing
a full path with '\' arguments produces incorrect results on Windows

Anyway users are advised to use relative paths for portability

testing/test_config.py

@@ -845,7 +845,7 @@ class TestOverrideIniArgs(object):
             assert inifile is None
 
     def test_addopts_before_initini(self, testdir, tmpdir, monkeypatch):
-        cache_dir = testdir.tmpdir.join('.custom_cache')
+        cache_dir = '.custom_cache'
         monkeypatch.setenv('PYTEST_ADDOPTS', '-o cache_dir=%s' % cache_dir)
         from _pytest.config import get_config
         config = get_config()