tests: Use subtests for JSON (#69)

Change JSON marshaling and unmarshaling tests to use subtests for
cleaner separation in output as well as for easier visual scanning.

assert_test.go

@@ -0,0 +1,45 @@
+// Copyright (c) 2020 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+package atomic
+
+import (
+	"encoding/json"
+	"errors"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+// Marks the test as failed if the error cannot be cast into the provided type
+// with errors.As.
+//
+//   assertErrorAsType(t, err, new(ErrFoo))
+func assertErrorAsType(t *testing.T, err error, typ interface{}, msgAndArgs ...interface{}) bool {
+	t.Helper()
+
+	return assert.True(t, errors.As(err, typ), msgAndArgs...)
+}
+
+func assertErrorJSONUnmarshalType(t *testing.T, err error, msgAndArgs ...interface{}) bool {
+	t.Helper()
+
+	return assertErrorAsType(t, err, new(*json.UnmarshalTypeError), msgAndArgs...)
+}