Unit tests test_util_spawn_background_* fail when run from make distcheck
Currently the unit tests
test_util_spawn_background_* assume that they are run from the Tor build directory (so as to find
test-child). This is not the case when running
make distcheck, so the test will fail. The path to
test-child can be found through
dirname(argv)/../test-child, but unit tests don't have access to
argv. Therefore one solution is to use autoconf to find the Tor build directory and base the path on this.
Originally reported by Sebastian Hahn.