ECM: attempt to fix KDEInstallDirsTest.relative_or_absolute on Windows

Summary:
Not passing CMAKE_INSTALL_PREFIX is a weird thing to do. The test shows
"Installing in ." and some values like KDE_INSTALL_FULL_EXECROOTDIR
become "/" which is considered relative on Windows.

The test that passes /usr to CMAKE_INSTALL_PREFIX actually passes on
Windows. Pass /tmp to the other test, remove the test without prefix.

Test Plan: Passes on Linux, not tested on Windows, CI will do that

Reviewers: kossebau, apol, cgiboudeaux

Reviewed By: apol

Subscribers: kde-frameworks-devel, kde-buildsystem

Tags: Frameworks, Build System

Differential Revision: https://phabricator.kde.org/D28409