tests: fix prints
- delete unnecessary newlines
- make sure formatted prints call the formatting routines

R=adg
CC=golang-dev
https://golang.org/cl/2225046
diff --git a/src/pkg/io/multi_test.go b/src/pkg/io/multi_test.go
index f5d2fad..3ecb7c7 100644
--- a/src/pkg/io/multi_test.go
+++ b/src/pkg/io/multi_test.go
@@ -83,6 +83,6 @@
 	}
 
 	if sink.String() != sourceString {
-		t.Error("expected %q; got %q", sourceString, sink.String())
+		t.Errorf("expected %q; got %q", sourceString, sink.String())
 	}
 }