Add integration test for filtering the stashes list by file path

The test shows that it doesn't work, the list is empty both when filtering by
file and by folder.

I could have added test cases for these to the unit tests in
stash_loader_test.go instead, but I don't find tests that need to mock git's
output very valuable, so I opted for an integration test even though it takes
much longer to run.

pkg/integration/tests/test_list.go

@@ -352,6 +352,7 @@ var tests = []*components.IntegrationTest{
 	stash.CreateBranch,
 	stash.Drop,
 	stash.DropMultiple,
+	stash.FilterByPath,
 	stash.Pop,
 	stash.PreventDiscardingFileChanges,
 	stash.Rename,