vfs: disable cache cleaner if --vfs-cache-poll-interval=0

And use this to disable the cleaner in the cache tests to make them
more reliable

vfs/cache.go

@@ -387,6 +387,10 @@ func (c *cache) clean() {
 //
 // doesn't return until context is cancelled
 func (c *cache) cleaner(ctx context.Context) {
+	if c.opt.CachePollInterval <= 0 {
+		fs.Debugf(nil, "Cache cleaning thread disabled because poll interval <= 0")
+		return
+	}
 	// Start cleaning the cache immediately
 	c.clean()
 	// Then every interval specified

vfs/cache_test.go

@@ -63,9 +63,12 @@ func TestCacheNew(t *testing.T) {
 	defer r.Finalise()
 
 	ctx, cancel := context.WithCancel(context.Background())
+	defer cancel()
 
-	c, err := newCache(ctx, r.Fremote, &DefaultOpt)
-	cancel() // kill the background cache cleaning as it interferes with the tests
+	// Disable the cache cleaner as it interferes with these tests
+	opt := DefaultOpt
+	opt.CachePollInterval = 0
+	c, err := newCache(ctx, r.Fremote, &opt)
 	require.NoError(t, err)
 
 	assert.Contains(t, c.root, "vfs")