tests: Generate Makefile-test-matrix.am.inc in $(srcdir)
tests: Generate Makefile-test-matrix.am.inc in $(srcdir)

Files that are generated manually by a maintainer and committed to git
should be in the srcdir, not the builddir.

Signed-off-by: Simon McVittie <0e1aea601907c1a85c2dc96e5ea37e23c58d2e9b@collabora.com>

flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak,flatpak/flatpak