Improve handling of @Parameter's defaultValue

[abelsromero]

What is this issue about?

• Bug report
• Feature request
• Question

Description

The Maven mojos can use defaultValue to set the default value of a configuration property. However, that is not respected in unit tests where we create a mojo with mockAsciidoctorMojo method.
Test work thanks to a combination of manually set defaults and code validations that should not be necessary.

Basic thing to do is parse annotations to set values in unit tests to get a 100% consistent testing experience. We can experiment with doing it in current mockAsciidoctorMojo or do some junit extension.