KSqueezedTextLabel: Add several autotests

Summary:
Tests most of the basic functionality and uncovers bugs for a few edge
cases. Testing some of the advanced features is skipped for now, there
might be more bugs lurking.

Checking the text elision in relation to font metrics is already being
covered by Qt's autotests.

Depends on D7162

Test Plan: Tests pass, except where marked as QEXPECT_FAIL.

Reviewers: Frameworks, dhaumann

Reviewed By: dhaumann

Subscribers: dhaumann

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D7163