Relocate binding body tests (#2086)

* Relocate binding body tests

Every test file should be related to a tested file.
Remove useless tests.

* Add github.com/stretchr/testify/require package

binding/json_test.go

@@ -0,0 +1,21 @@
+// Copyright 2019 Gin Core Team. All rights reserved.
+// Use of this source code is governed by a MIT style
+// license that can be found in the LICENSE file.
+
+package binding
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestJSONBindingBindBody(t *testing.T) {
+	var s struct {
+		Foo string `json:"foo"`
+	}
+	err := jsonBinding{}.BindBody([]byte(`{"foo": "FOO"}`), &s)
+	require.NoError(t, err)
+	assert.Equal(t, "FOO", s.Foo)
+}