tests: Add simple test for summary file caching

tests: Add simple test for summary file caching

This test would have actually passed before the summary file caching
changes (in the previous few commits) were added, as the `summary.sig`
essentially acted as the ETag for the summary file, and itself wasn’t
updated on disk if it didn’t change when querying the server.

Actually testing that the HTTP caching headers are working to reduce
HTTP traffic would require test hooks into the pull code or the
trivial-httpd server, neither of which I have the time to add at the
moment.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>