Test more probative URL in TestUnreachableSource

manager_test.go

@@ -785,12 +785,15 @@ func TestSignalHandling(t *testing.T) {
 func TestUnreachableSource(t *testing.T) {
 	// If a git remote is unreachable (maybe the server is only accessible behind a VPN, or
 	// something), we should return a clear error, not a panic.
+	if testing.Short() {
+		t.Skip("Skipping slow test in short mode")
+	}
 
 	sm, clean := mkNaiveSM(t)
 	defer clean()
 
-	id := mkPI("golang.org/notareal/repo").normalize()
-	_, err := sm.ListVersions(id)
+	id = mkPI("github.com/golang/notexist").normalize()
+	err = sm.SyncSourceFor(id)
 	if err == nil {
 		t.Error("expected err when listing versions of a bogus source, but got nil")
 	}