test(scp): test spaces after completing filenames ending with '\'

The test has not been prepared because the ending spaces cannot be
tested with the typical comparison of `completion == "..."` or
`completion == [...]`, but it turned out that existing teset cases
uses `completion.output` to check suffixing spaces.  This patch
implements the test using `completion.output`.

#1397 (comment)

Author: akinomyoga

test/t/test_scp.py

@@ -1,4 +1,5 @@
 import os
+import sys
 from itertools import chain
 
 import pytest
@@ -158,6 +159,28 @@ def test_xfunc_remote_files(self, bash):
             "shared/default/foo.d/",
         ]
 
+    @pytest.fixture
+    def tmpdir_backslash(self, request, bash):
+        if sys.platform.startswith("win"):
+            pytest.skip("Filenames not allowed on Windows")
+
+        tmpdir, _, _ = prepare_fixture_dir(
+            request, files=["local_path-file\\"], dirs=[]
+        )
+        return tmpdir
+
+    def test_local_path_ending_with_backslash(self, bash, tmpdir_backslash):
+        completion = assert_complete(
+            bash, "scp local_path-", cwd=tmpdir_backslash
+        )
+        assert completion.output == r"file\\ "
+
+    def test_remote_path_ending_with_backslash(self, bash):
+        assert_bash_exec(bash, "ssh() { echo 'hypothetical\\'; }")
+        completion = assert_complete(bash, "scp remote_host:hypo")
+        assert_bash_exec(bash, "unset -f ssh")
+        assert completion.output == r"thetical\\\\ "
+
     @pytest.fixture
     def tmpdir_mkfifo(self, request, bash):
         tmpdir, _, _ = prepare_fixture_dir(request, files=[], dirs=[])