test(scp): Add unit tests for getting remote files

This tests the current behaviour of the
xfunc_scp_compgen_remote_files function, including escaping, by
introducing a fixture to mock ssh invocation on the local host.

test/fixtures/_comp_compgen_xfunc_scp_remote_files/bin/ssh

@@ -0,0 +1,10 @@
+#!/bin/sh
+for arg in "$@"; do
+    case "$arg" in
+	-o)
+		shift 2;;
+	local)
+		shift 1;;
+    esac
+done
+$@

test/t/unit/Makefile.am

@@ -36,7 +36,8 @@ EXTRA_DIST = \
 	test_unit_realcommand.py \
 	test_unit_split.py \
 	test_unit_unlocal.py \
-	test_unit_xfunc.py
+	test_unit_xfunc.py \
+	test_unit_xfunc_scp_remote_files.py
 
 all:
 

test/t/unit/test_unit_xfunc_scp_remote_files.py

@@ -0,0 +1,33 @@
+import pytest
+
+from conftest import assert_bash_exec, bash_env_saved
+
+
+@pytest.mark.bashcomp(cmd="ssh", ignore_env=r"^\+COMPREPLY=")
+class TestUnitXfuncScpRemoteFiles:
+    @pytest.fixture(scope="class")
+    def ssh_path(self, request, bash):
+        with bash_env_saved(bash) as bash_env:
+            bash_env.write_variable(
+                "PATH",
+                "$PWD/_comp_compgen_xfunc_scp_remote_files/bin:$PATH",
+                quote=False,
+            )
+            yield
+
+    def test_1(self, bash, ssh_path):
+        completions = (
+            assert_bash_exec(
+                bash,
+                r'cur=local:shared/default/ _comp_compgen -ax scp remote_files; printf "%s\n" "${COMPREPLY[@]}"; unset cur',
+                want_output=True,
+            )
+            .strip()
+            .splitlines()
+        )
+        assert completions == [
+            "shared/default/bar ",
+            r"shared/default/bar\\\ bar.d/",
+            "shared/default/foo ",
+            "shared/default/foo.d/",
+        ]