tests: Work around summary mtime cache issue (for 1.8 branch)

This adds a sleep(1) before each summary update (if there is a
pre-existing summary file). This avoids issues where a new summary
file get the same mtime (in seconds precision).

This is kind of a hacky work around, but it is good enought to get
the flatpak-1.8 branch working with latest ostree, and master has a better
fix already.

tests/libtest.sh

@@ -360,6 +360,10 @@ update_repo () {
         collection_args=
     fi
 
+    if test -f repos/${REPONAME}/summary; then
+        sleep 1 # ensure we get a new timestamp on the summary files
+    fi
+
     ${FLATPAK} build-update-repo ${collection_args} ${GPGARGS:-${FL_GPGARGS}} ${UPDATE_REPO_ARGS-} repos/${REPONAME}
 }