As of early April 2020, running the Syncthing test suite on the Gentoo ppc64
buildhost fails due to TestIssue5063 timing out. Increase the time-out
threshold for this test to allow it to pass.

Other arches we currently support (i.e. amd64, arm and x86) shouldn't be
affected by the change because thay have always passed so far anyway.

--- a/lib/model/model_test.go
+++ b/lib/model/model_test.go
@@ -914,7 +914,7 @@
 	}()
 	select {
 	case <-finished:
-	case <-time.After(10 * time.Second):
+	case <-time.After(20 * time.Second):
 		pprof.Lookup("goroutine").WriteTo(os.Stdout, 1)
 		t.Fatal("Timed out before all devices were added")
 	}