test: Fix testenv-root lingering in build dir

Signed-off-by: Jacques de Laval <jacques@de-laval.se>

test/setup-root.sh

@@ -2,14 +2,7 @@
 
 set -eu
 
-if [ ! -d test-root ]; then
-    mkdir -p test-root/bin
-    # shellcheck disable=SC2154
-    cp "${srcdir}/test-root/bin/chrootsetup.sh" test-root/bin
-    cp "${srcdir}/test-root/busybox-x86_64.md5" test-root/
-    cp "${srcdir}/test-root/Makefile" test-root/
-fi
-
 # shellcheck disable=SC2154
-make -C "$top_builddir" DESTDIR="$(pwd)/test-root/" install
-FINITBIN="$(pwd)/$top_builddir/src/finit" make -C test-root/
+make -C "$top_builddir" DESTDIR="$TESTENV_ROOT" install
+# shellcheck disable=SC2154
+FINITBIN="$(pwd)/$top_builddir/src/finit" DEST="$TESTENV_ROOT" make -f "$srcdir/testenv-root.mk"