meson build test phase writes and leaves files in /tmp

[pacho2]

As reported downstream at:
https://bugs.gentoo.org/659446

Running of the tests leaves some directories and files in /tmp
/tmp/32x32/ /tmp/destdir/ /tmp/origin/ /tmp/foo.png

Debian looks to try to avoid that issue setting XDG_RUNTIME_DIR to a different value:
https://salsa.debian.org/pkgutopia-team/appstream-glib/-/blob/master/debian/rules?ref_type=heads#L40

But, at least on my Gentoo system, it seems that it is not honored and continues to write on /tmp

Thanks

[hughsie]

A merge request would be very welcome.