fix(logger): make MockLogger lock the buffer implicitly

[thp-canonical]

This has been ported from a similar change in Pebble:

• fix(logger): Make MockLogger threadsafe pebble#441

The explicit logging feature was implemented in snapd in commit 16c3680b78d, but this requires a developer to know when another goroutine might be using the logger, and using WithLoggerLock() explicitly. In addition, as WithLoggerLock() acquires the global logger lock, trying to log from the callback of WithLoggerLock() might result in a deadlock (but the testing code right now isn't doing this, of course).

Instead of requiring explicit locking, this change makes it so that Write(), String() and Reset() access to the bytes.Buffer is mutex-protected.

As logger.MockLogger() is only used in unit test code, locking an otherwise uncontested mutex for Write() should hopefully not cause any noticeable slowdown, while making it impossible to accidentally use the mock logger in a thread-unsafe way.