Entry guard unittest (#12207) fail in out-of-tree builds

In the unittests introduced by #12207 (moved), I'm reading a file containing some fake descriptors.

The file is at /src/test/test_descriptors.txt and when I'm loading it in the code, I do:

static void *
fake_network_setup(const struct testcase_t *testcase)
{
  /* This is the file containing our test descriptors. */
  const char *fname = BUILDDIR "/src/test/test_descriptors.txt";

But apparently, the BUILDDIR macro was completely wrong there, since I was looking for the root of the source directory, whereas BUILDDIR is the directory we are building (who would have thought). Apparently, this breaks the unittests in the out-of-tree builds of jenkins.

We should probably find the right macro to use there. I didn't find a SRCDIR in config.log but maybe autotools already have something for us?