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 <smcv@collabora.com>
(cherry picked from commit 647c51380c13b72adbd67e0aa83b3a2979f5c6a8)