Improve code coverage for metadata generator classes

[gaurabdg]

Classes present in https://github.com/checkstyle/checkstyle/tree/master/src/main/java/com/puppycrawl/tools/checkstyle/meta

100% code coverage is required

TODO:

• Initialize and setup UT backbone for JavadocScraper
• Line Cov - 1.00 | Current: 0.98
• Branch Cov - 1.00 | Current: 0.88