glib-tap: Add missing mkdir for .test generation rule

We can’t guarantee that the builddir we’re generating in actually exists
— if doing a clean build with builddir ≠ srcdir, and there are no other
rules which generate build products in builddir, the .test generation
rule can fail. This happens for flatpak-builder.git for me.

Try to avoid that by explicitly creating the builddir.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=790785