Add unit tests for suffix generation for downloads

The pushpin is my standard high unicode value codepoint. Some things
handle umlauts etc fine but choke on higher values.

The behavior seems fine enough I guess. Some people might
prefer " (2)" instead of "_2". Also maybe we should start at 1 instead
of 2?

qutebrowser/browser/downloads.py

@@ -500,6 +500,9 @@ class AbstractDownloadItem(QObject):
         """
         assert self._filename is not None
         path, file = os.path.split(self._filename)
+        # Pull out filename extension which could be a two part one like
+        # `tar.gz`. Use a more restrictive character set for the first part of
+        # a two part extension to avoid matching numbers.
         match = re.fullmatch(r'(.+?)((\.[a-z]+)?\.[^.]+)', file)
         if match:
             base, suffix = match[1], match[2]

tests/unit/browser/test_downloads.py

@@ -2,6 +2,8 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
+import os
+
 import pytest
 
 from qutebrowser.browser import downloads, qtnetworkdownloads
@@ -113,6 +115,33 @@ def test_sanitized_filenames(raw, expected,
     assert item._filename.endswith(expected)
 
 
+@pytest.mark.parametrize('filename, expected', [
+    ("noext", "noext_2"),
+    ("simple.gif", "simple_2.gif"),
+    ("twoparts.tar.gz", "twoparts_2.tar.gz"),
+    ("many.dots.in.the.name", "many.dots.in_2.the.name"),
+    ("a. space.gif", "a. space_2.gif"),
+    ("non-ascii-📍.gif", "non-ascii-📍_2.gif"),
+    ("non-ascii.📍", "non-ascii_2.📍"),
+    ("non-ascii.📍.gif", "non-ascii.📍_2.gif"),
+    ("numbers_22.10.05.jpeg", "numbers_22.10.05_2.jpeg"),
+    ("Sentance..gif", "Sentance._2.gif"),
+])
+def test_generated_filename_suffix(
+    filename, expected, config_stub, download_tmpdir, monkeypatch
+):
+    manager = downloads.AbstractDownloadManager()
+    item = downloads.AbstractDownloadItem(manager=manager)
+
+    # Abstract methods
+    item._ensure_can_set_filename = lambda *args: True
+    item._after_set_filename = lambda *args: True
+
+    item._set_filename(filename)
+    item._find_next_filename()
+    assert os.path.basename(item._filename) == expected
+
+
 class TestConflictingDownloads:
 
     @pytest.fixture