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/ignore/cache_test.go

@@ -12,6 +12,13 @@ import (
 )
 
 func TestCache(t *testing.T) {
+	fc := new(fakeClock)
+	oldClock := clock
+	clock = fc
+	defer func() {
+		clock = oldClock
+	}()
+
 	c := newCache(nil)
 
 	res, ok := c.get("nonexistent")
@@ -52,11 +59,11 @@ func TestCache(t *testing.T) {
 
 	// Sleep and access, to get some data for clean
 
-	time.Sleep(500 * time.Millisecond)
+	*fc += 500 // milliseconds
 
 	c.get("true")
 
-	time.Sleep(100 * time.Millisecond)
+	*fc += 100 // milliseconds
 
 	// "false" was accessed ~600 ms ago, "true" was accessed ~100 ms ago.
 	// This should clean out "false" but not "true"
@@ -75,3 +82,11 @@ func TestCache(t *testing.T) {
 		t.Errorf("item should have been cleaned")
 	}
 }
+
+type fakeClock int64 // milliseconds
+
+func (f *fakeClock) Now() time.Time {
+	t := time.Unix(int64(*f)/1000, (int64(*f)%1000)*int64(time.Millisecond))
+	*f++
+	return t
+}