Fix common.user test

The username was being cached from previous tests, so clear it before
running this test.

tests/test_segments.py

@@ -73,6 +73,7 @@ class TestCommon(TestCase):
 		new_psutil = new_module('psutil', Process=lambda pid: Args(username='def'))
 		pl = Pl()
 		with replace_env('USER', 'def') as segment_info:
+			common.username = False
 			with replace_attr(common, 'os', new_os):
 				with replace_attr(common, 'psutil', new_psutil):
 					with replace_attr(common, '_geteuid', lambda: 5):