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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