Ensure index cache entry written in test

Ensure index cache entry written in test

Now that index cache files are written in the background, one test in
index-cache.exp is racy -- it assumes that the cache file will have
been written during startup.

This patch fixes the problem by introducing a new maintenance command
to wait for all pending writes to the index cache.

Approved-By: Simon Marchi <simon.marchi@efficios.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>