Add a testlogger util package for testing go-logr.

Signed-off-by: Matt Moyer <moyerm@vmware.com>

internal/testutil/testlogger/testlogger.go

@@ -0,0 +1,61 @@
+/*
+Copyright 2020 VMware, Inc.
+SPDX-License-Identifier: Apache-2.0
+*/
+
+// Package testlogger implements a logr.Logger suitable for writing test assertions.
+package testlogger
+
+import (
+	"bytes"
+	"log"
+	"strings"
+	"sync"
+	"testing"
+
+	"github.com/go-logr/logr"
+	"github.com/go-logr/stdr"
+)
+
+// Logger implements logr.Logger in a way that captures logs for test assertions.
+type Logger struct {
+	logr.Logger
+	t      *testing.T
+	buffer syncBuffer
+}
+
+// New returns a new test Logger.
+func New(t *testing.T) *Logger {
+	res := Logger{t: t}
+	res.Logger = stdr.New(log.New(&res.buffer, "", 0))
+	return &res
+}
+
+// Lines returns the lines written to the test logger.
+func (l *Logger) Lines() []string {
+	l.t.Helper()
+	l.buffer.mutex.Lock()
+	defer l.buffer.mutex.Unlock()
+
+	// Trim leading/trailing whitespace and omit empty lines.
+	var result []string
+	for _, line := range strings.Split(l.buffer.buffer.String(), "\n") {
+		line = strings.TrimSpace(line)
+		if line != "" {
+			result = append(result, line)
+		}
+	}
+	return result
+}
+
+// syncBuffer synchronizes access to a bytes.Buffer.
+type syncBuffer struct {
+	mutex  sync.Mutex
+	buffer bytes.Buffer
+}
+
+func (s *syncBuffer) Write(p []byte) (n int, err error) {
+	s.mutex.Lock()
+	defer s.mutex.Unlock()
+	return s.buffer.Write(p)
+}