[FEATURE] Logging to testing logger

[mitar]

New feature request checklist

• I have updated go-smtp-mock to the latest version
• I have read the Contribution Guidelines
• I have read the documentation
• I have searched for existing GitHub issues

Feature description

Currently this package supports logging to stdout. Which works well, but I think inside unit test it would be even better if it would support logging to testing.Logf. Then it is easier to see chronological order of logging messages, when other parts of your unit test are using testing.Logf. They are also shown only if the test fails.

I think API could be an option to ConfigurationAttr, something like LogToTesting *testing.T and you would pass *testing.T instance to it.

Alternative would be that I could pass my own "logger" instance and then I could make my own adapter.

[bestwebua]

@mitar I'll take for develop this feature next week.