Help KSldTest to find its helper binaries with latest ECM's bin/ collection

With all libraries, plugins and executables built into builddir's
toplevel "bin" directory, the working directory on test execution no
longer matches the binary location.

https://community.kde.org/Guidelines_and_HOWTOs/Making_apps_run_uninstalled
recommends to just use QDir::setCurrent(QCoreApplication::applicationDirPath());