test: move to koji-osbuild-tests package

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.

test/copy-creds.sh

@@ -7,7 +7,8 @@ if [ $UID != 0 ]; then
   exit 1
 fi
 
-TEST_DATA=${TEST_DATA:-test/data}
+TEST_PATH=${1:-test}
+TEST_DATA=${TEST_PATH}/data
 SHARE_DIR=${SHARE_DIR:-/tmp/osbuild-composer-koji-test}
 
 mkdir -p "${SHARE_DIR}"