Add xfail tests for URL subtraction with empty segments

#1389 (comment)
#1388 (comment)

tests/test_url.py

@@ -66,11 +66,6 @@ def test_str():
     [
         ("http://example.com/path/to", "http://example.com/", "path/to"),
         ("http://example.com/path/to", "http://example.com/spam", "path/to"),
-        (
-            "http://example.com/////path/////to",
-            "http://example.com/////spam",
-            "path/to",
-        ),
         ("http://example.com/this/is/a/test", "http://example.com/this/", "is/a/test"),
         (
             "http://example.com/this/./is/a/test",
@@ -103,6 +98,28 @@ def test_sub(target: str, base: str, expected: str):
     assert result_url == expected_url
 
 
+@pytest.mark.xfail(reason="Empty segments are not preserved")
+@pytest.mark.parametrize(
+    ("target", "base", "expected"),
+    [
+        (
+            "http://example.com/////path/////to",
+            "http://example.com/////spam",
+            "path/////to",
+        ),
+        (
+            "http://example.com////path/////to",
+            "http://example.com/////spam",
+            "..//path/////to",
+        ),
+    ],
+)
+def test_sub_empty_segments(target: str, base: str, expected: str):
+    expected_url = URL(expected)
+    result_url = URL(target) - URL(base)
+    assert result_url == expected_url
+
+
 def test_sub_with_different_schemes():
     expected_error_msg = "Both URLs should have the same scheme"
     with pytest.raises(ValueError, match=expected_error_msg):