lib/ignore, lib/model: Use an interface to detect file changes, improving tests

This solves the erratic test failures on model.TestIgnores by ensuring
that the ignore patterns are reloaded even in the face of unchanged
timestamps.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/4208

lib/scanner/walk_test.go

@@ -54,7 +54,7 @@ func init() {
 }
 
 func TestWalkSub(t *testing.T) {
-	ignores := ignore.New(false)
+	ignores := ignore.New()
 	err := ignores.Load("testdata/.stignore")
 	if err != nil {
 		t.Fatal(err)
@@ -90,7 +90,7 @@ func TestWalkSub(t *testing.T) {
 }
 
 func TestWalk(t *testing.T) {
-	ignores := ignore.New(false)
+	ignores := ignore.New()
 	err := ignores.Load("testdata/.stignore")
 	if err != nil {
 		t.Fatal(err)