Add proper public testing API

[JeanMertz]

Testing

This package contains a public testing API you can use if you need to assert
a log entry exists.

// TestNew calls New, but returns both the logger, and an observer that can be
// used to fetch and compare delivered logs.
TestNew(tb testing.TB, options ...zap.Option) (*zap.Logger, *observer.ObservedLogs)

// TestNewWithLevel is equal to TestNew, except that it takes an extra argument,
// dictating the minimum log level required to record an entry in the recorder.
TestNewWithLevel(tb testing.TB, level zapcore.LevelEnabler, options ...zap.Option) (*zap.Logger, *observer.ObservedLogs)

see testing.go for more details.