Use newer add_test SIGNATURE, as needed with min ECM >= 5.38

d18f3a97 bumped min ECM from 3.18 to 5.68, which triggered the breakage.

See https://api.kde.org/ecm/kde-module/KDECMakeSettings.html#build-settings for the reason, the test binary is no longer placed in the same dir, thus we need to use add_test with the variant that takes a target. See https://cmake.org/cmake/help/latest/command/add_test.html#command:add_test

BTW, there a lots of nullptr changes possible, okay if I let clang-tidy fix that for us& push directly?