Test suite: improve --late-patch

I have wasted a lot of time by failing to either remember that I must provide both source and destination in the --late-patch file, or by forgetting to add new files I change to it. With this new version I expect --late-patch without argument to cover most usage of this option.

This is its new documentation:

  --late-patch
  --late-patch=FILE
                     Copies files into the VM before running the test suite,
                     which often can avoid a rebuild.
                     FILE is a text file where each line contains tab-separated
                     source and destination. If only a source is given, it is
                     assumed to be a file inside config/chroot_local-includes,
                     and the corresponding destination is inferred. Lines with
                     a leading \"#\" are ignored.
                     FILE is optional, and if not given we copy in all files in
                     config/chroot_local-includes that have changed since the
                     commit the system under testing was built from, including
                     untracked files.

Closes #20256 (closed)