Instead of just using the "latest" container everywhere, which will
change every time a new release is made, add a build argument to
specify the version and then match that version to the host in all
the build scripts. This will make it possible to use the tests for
gating, and ensure that we test the plugins on the OS version that
is targeted.
This is similar to how other osbuild packages are testing: everything
that's needed for testing is included in the tests package or a
dependency of it. The test runner then runs every executable in
/usr/libexec/tests/<packagename>. This gives a simple test API to
projects depending on this package (notably osbuild-composer).
The local development workflow described in HACKING.md is meant to
continue to work. To ensure this, all relevant scripts gained a
TEST_DATA variable, which defaults to `./test`, but is set from $1 to
the installed path from integration.sh.
Move the container directory, containing the container definitions
for all the test containers, to test/, where all the other test-
related files are located (with the exception of `Schutzbot`).
Use `test/build-container.sh` to build the container, instead of
replicating that in `test-integration.sh`.
