KFormat: Add unit tests for mili/micro SI prefixes

Summary:
Add test cases for KFormat::formatValue(...) with value smaller than
1.0, with fixed and automatic SI prefix.

Reviewers: Frameworks, kossebau, astippich

Reviewed By: astippich

Subscribers: astippich, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D14121