test: use absolute path in run-builder.sh

In case `TEST_PATH` was not specified as command line argument,
it was falling back to `test`. Make the latter an absolute path,
by pre-pending `PWD`, otherwise podman complains about the name
of the volume.

test/run-builder.sh

@@ -1,7 +1,7 @@
 #!/usr/bin/bash
 set -euo pipefail
 
-TEST_PATH=${2:-test}
+TEST_PATH=${2:-${PWD}/test}
 SHARE_DIR=${SHARE_DIR:-/tmp/osbuild-composer-koji-test}
 DATA_DIR=${DATA_DIR:-/var/tmp/osbuild-koji-data}