acme/autocert: clean up temporary directories in TestDirCache()

Change-Id: I5de781adf4071158404da6252d06e4c2c33f298b
Reviewed-on: https://go-review.googlesource.com/47356
Reviewed-by: Alex Vaghin <ddos@google.com>

acme/autocert/cache_test.go

@@ -21,6 +21,7 @@ func TestDirCache(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	defer os.RemoveAll(dir)
 	dir = filepath.Join(dir, "certs") // a nonexistent dir
 	cache := DirCache(dir)
 	ctx := context.Background()