Document how to integrate GTest into your project

We are missing the documentation on how to use GTest inside a project.

While we mention in passing gtester and gtester-report, the reality is
that we don't tell anybody how to use them inside their own build
system. Additionally, gtester and gtester-report are deprecated and kind
of abandoned, in favour of tools native to common build systems, such as
the TAP harness in Autotools, and the test harness in Meson.

See also:

  https://stackoverflow.com/questions/19958861/how-to-properly-set-up-glib-testing-framework-with-autotools

https://bugzilla.gnome.org/show_bug.cgi?id=788561