test SystemInformation class

Summary:
- has a new Config struct, encapsulates data we need to mangle for
  testing
- new completeness state to make sure lsb_release has returned by the time
  we read data in the test
- os-release fixture
- lsb_release double

the os-release test has expected failure right now because the current
parser is in fact broken. needs porting to KOsRelease

Test Plan: builds && passes

Reviewers: #plasma, apol

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22137