Fix build of plugin and tests with custom target suffix

In my previous pull request I introduced a TARGET_SUFFIX variable, but missed adjusting the plugin and tests subdirectories. This happened, because
1.) in my native test builds, libconnman-qt4 was already installed on the system, therefore hiding the linker issue.
2.) in OpenEmbedded cross-builds, the options notests and noplugin were set.
These issues don't appear if TARGET_SUFFIX is either unset or set to one of qt4 or qt5.