test-composefs: Sync flow with other tests

I am not sure why this is failing on older Debian systems,
but I'm wildly guessing that something being done in `libtest.sh`
is setting up automake in a way that we need.  This is done
in other tests.

Or maybe it's the missing `$CMD_PREFIX`?  Let's see...

tests/test-composefs.sh

@@ -17,12 +17,13 @@
 
 set -euo pipefail
 
-if ! ostree --version | grep -q -e '- composefs'; then
+. $(dirname $0)/libtest.sh
+
+if ! ${CMD_PREFIX} ostree --version | grep -q -e '- composefs'; then
     echo "1..0 #SKIP no composefs support compiled in"
     exit 0
 fi
 
-. $(dirname $0)/libtest.sh
 
 setup_test_repository "bare-user"