Tests: Fix backend loading for tests

Using $<TARGET_FILE> we receive the entire plugin file path, leading to
errors such as:

kf.coreaddons: no metadata found in "kpmcore//home/awilcox/Code/KDE/kpmcore/bin/kpmcore/pmdummybackendplugin.so" "The shared library was not found."

Using $<TARGET_FILE_NAME> instead we are able to have just the plugin
name, which does load properly.  The tests now pass.