This fixes build with builddir ≠ srcdir and a non-existent builddir: the
$(builddir)/tests/ directory was not being created by the time the .test
files were being written.
This will be submitted upstream to glib-tap.mk in GLib.
Signed-off-by: Philip Withnall <withnall@endlessm.com>
Closes: #68
Approved by: alexlarsson