user/opencc: enable tests

[bjin]

Description

This PR updates opencc to use system library (instead of bundled ones in repository), and enable tests for "check" phase. Another package "tclap" was also added with tests enabled.

Checklist

Before this pull request is reviewed, certain conditions must be met.

The following must be true for all changes:

• I have read CONTRIBUTING.md

The following must be true for template/package changes:

• I have read Packaging.md
• I have built and tested my changes on my machine

The following must be true for new package submissions:

• I will take responsibility for my template and keep it up to date

[bjin]

addressed all comments, squashed and rebased

[bjin]

addressed all comments, and squashed. Not rebased this time, so comparing commits should reflects all changes addressing review comments