In our Akademy presentation, Kévin and I showed the importance for a better developer story to be able to work on a KDE module without having to install it. Running unittests and running applications without installing the module at all is possible, it turns out, it just needs a bit of effort to set things up correctly.
Once you require ECM version 5.38 (using find_package(ECM 5.38)), your libraries, plugins and executables will all go to the builddir's "bin" directory, instead of being built in the builddir where they are defined.
After doing that, see if this works:
Oops, usually it doesn't work. Here's what you might have to do to fix things.
There are two issues I didn't solve yet: trader queries that should find uninstalled desktop files, and QML components, like in kirigami. It seems that the only solution for the latter is to reorganize the source dir to have the expected layout "org/kde/kirigami.2/*"?
Update: this howto is now a wiki page.