AdduserCommon: Fix sys_name_regex to match documentation (tests added)

This is a modified version of !84 (closed) with improvements and additions in the valid_username.t test script.

The first two commits here show that expected tests fail when valid_username.t is switched to first checking against adduser defaults.

I then cherry-picked the commit from @guillem that fixes these failures and pushed it to show that the pipeline starts to pass.

Commit messages and pipeline output have more details, of course.

Ping me with any questions!