Improved testing

[Thomasdezeeuw]

• [x] Testing with flaky writer that sometimes returns an error.
• [x] get 100% coverage.

[Thomasdezeeuw]

After the rewrite we're almost at 100% coverage, currently at 98%. Three lines are currently not covered:

• https://github.com/Thomasdezeeuw/logger/blob/master/event.go#L59, logger.Tags.MarshalJSON never returns an error, but it's their if it ever changes.
• https://github.com/Thomasdezeeuw/logger/blob/master/log.go#L226, only happens if logger.Thumbstone is called as main function, so also not possible.
• https://github.com/Thomasdezeeuw/logger/blob/master/writers.go#L49, untested but only returns the error.

So I would say the 100% coverage is complete.

[Thomasdezeeuw]

Fixed by commit https://github.com/Thomasdezeeuw/logger/commit/5f9f418c5d55e6392b0f984aa60977452e8eb3a4.