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.
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