[Minor] test: remove hard-coded build directory

[Minor] test: remove hard-coded build directory

* Reproducible build: avoid defining the BUILDROOT macro and avoid
  embedding the build directory in the test image. Instead rely on the
  test files being present next to the executable.
* I considered using g_test_build_filename, available since GLib 2.38
  (available on all supported platforms, RHEL 7, Debian, Ubuntu, etc.),
  but decided against it because it would require setting the
  G_TEST_SRCDIR or G_TEST_BUILDDIR environment variables. Therefore this
  patch simply parses argv0 directly.