tests/Makefile.am.inc: Fix distcheck error
tests/Makefile.am.inc: Fix distcheck error

Currently `make distcheck` fails with:

PATH=$(cd . && pwd):${PATH} tests/make-test-runtime.sh
tests/runtime-repo org.test.Platform ""
/bin/bash: tests/make-test-runtime.sh: No such file or directory

This was caused by commit b5d86fe90 and is fixed by this commit.

matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,flatpak/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,flatpak/flatpak,flatpak/flatpak