Before, after a sync, only file modtimes were updated when not using
--copy-empty-src-dirs. This ensures modtimes are updated to match the source
folder, regardless of copyEmptySrcDir. The flag --no-update-dir-modtime
(which previously did nothing) will disable this.
This adds tests to check dir modtimes are updated from source
when syncing even if they've changed in the destination.
This should work both with and without --copy-empty-src-dirs.