Ensure that qsqlite3.so is found in the builddir while running tests.

* Port KF5AkonadiMacros to ecm_add_test
* Set the output directory (in the builddir) for the plugin, as
documented on https://community.kde.org/Guidelines_and_HOWTOs/Making_apps_run_uninstalled

Result:
QT_DEBUG_PLUGINS=1 shows
QLibraryPrivate::load loaded library ".../akonadi/build-asan/bin/sqldrivers/libqsqlite3.so"