internal: use t.Errorf in test output with formatting directives

Change-Id: Ic12698e35565201310235b439558ae57e3813ae6
Reviewed-on: https://go-review.googlesource.com/51950
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/internal/catmsg/varint_test.go b/internal/catmsg/varint_test.go
index a079c77..04d881d 100644
--- a/internal/catmsg/varint_test.go
+++ b/internal/catmsg/varint_test.go
@@ -109,7 +109,7 @@
 			t.Run(fmt.Sprintf("%s:%q", f.name, tc.enc), func(t *testing.T) {
 				x, size, err := f.decode(tc.enc)
 				if err != tc.err {
-					t.Error("err = %q; want %q", err, tc.err)
+					t.Errorf("err = %q; want %q", err, tc.err)
 				}
 				if size != tc.size {
 					t.Errorf("size = %d; want %d", size, tc.size)