http2: fix nits found by vet

Change-Id: Ic4a3cc83d0f76a6a0d5e90b256761877151ab1e9
Reviewed-on: https://go-review.googlesource.com/18279
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

http2/server_test.go

@@ -2829,7 +2829,7 @@ func TestConfigureServer(t *testing.T) {
 			t.Errorf("%s: err = %v; want substring %q", tt.name, err, tt.wantErr)
 		}
 		if err == nil && !tt.in.TLSConfig.PreferServerCipherSuites {
-			t.Error("%s: PreferServerCipherSuite is false; want true", tt.name)
+			t.Errorf("%s: PreferServerCipherSuite is false; want true", tt.name)
 		}
 	}
 }

http2/transport_test.go

@@ -203,7 +203,7 @@ func TestTransportGroupsPendingDials(t *testing.T) {
 		}
 		return nil
 	}); err != nil {
-		t.Error("State of pool after CloseIdleConnections: %v", err)
+		t.Errorf("State of pool after CloseIdleConnections: %v", err)
 	}
 }