test: Be smarter about detecting the mkosi configuration directory

test: Be smarter about detecting the mkosi configuration directory

Instead of always looking up two directories from the
test/integration-tests/meson.build file, let's search in up to 4
parent directories from the given meson project source root. This
allows us to just pass in meson.project_source_root() to
integration-test-wrapper.py instead of having to pass in a fixed
relative offset from the current meson file.

It'll also allow us to install the integration tests and mkosi
configuration in the future without breaking the standalone
integrationt tests functionality;