Refactor collapse user logic

Use simple loop for when we aren't collapsing users.
Add test which covers case when users are deleted.

Signed-off-by: Will Beason <willbeason@gmail.com>

test/Wikiq_Unit_Test.py

@@ -177,6 +177,19 @@ class WikiqTestCase(unittest.TestCase):
         baseline = pd.read_table(tester.baseline_file)
         assert_frame_equal(test, baseline, check_like=True)
 
+    def test_WP_collapse_user(self):
+        tester = WikiqTester(IKWIKI, "collapse_user")
+
+        try:
+            tester.call_wikiq("--collapse-user")
+        except subprocess.CalledProcessError as exc:
+            self.fail(exc.stderr.decode("utf8"))
+
+        copyfile(tester.call_output, tester.test_file)
+        test = pd.read_table(tester.test_file)
+        baseline = pd.read_table(tester.baseline_file)
+        assert_frame_equal(test, baseline, check_like=True)
+
     def test_noargs(self):
         tester = WikiqTester(SAILORMOON, "noargs", in_compression="7z")