...mostly.
This creates and uses an override for the `serenity_test()` function, so
that Lagom can make use of the existing `Tests/LibFoo/CMakeLists.txt`
files instead of having to GLOB for test source files and manually copy
any data files.
Some GLOBs remain but this is most of them.
Co-authored-by: Andrew Kaster <akaster@serenityos.org>