test-pull-summary-sigs: Set timestamps to serve expected files

test-pull-summary-sigs: Set timestamps to serve expected files

If this is not done, the test can fail when the temporary directory is
a tmpfs: for example this happens during build-time testing with /var/tmp
on tmpfs or TEST_TMPDIR pointing to a tmpfs, or installed-tests with
gnome-desktop-testing-runner allocating the test directory on a tmpfs.

In particular, many of Debian's official autobuilders now do the entire
build and test procedure in a chroot hosted on a tmpfs, to improve build
performance and prevent fsync overhead.

In this situation, it appears that overwriting summary.sig with a copy
of summary.sig.2 is not sufficient for the web server to tell the
libostree client that it needs to be re-downloaded. I'm not completely
sure why, because tmpfs does appear to have sub-second-resolution
timestamps, but forcing a distinct mtime is certainly enough to
resolve it.

Resolves: https://github.com/ostreedev/ostree/issues/2245
Bug-Debian: https://bugs.debian.org/975418
Signed-off-by: Simon McVittie <smcv@collabora.com>
Forwarded: https://github.com/ostreedev/ostree/pull/2246

Gbp-Pq: Name test-pull-summary-sigs-Set-timestamps-to-serve-expected-f.patch