delete reference files not used by testo (or whitelisted) (#911)

delete reference files not used by testo (or whitelisted) (#911)

* delete reference files not used by testo.

the script tools/obsolete_unused_reference_files does this,
altough it might be delicate.
note any unused files in the reference tree will be deleted!

* restore used alternate reference file

* restore and whitelist some reference files.

* restore and whitelist a reference readme file.

* Revert "restore and whitelist a reference readme file."

This reverts commit 496fbf58d9b083863ae5011db4f8e48e044af546.

* use 'sort -u' instead of 'sort | uniq'.

* whitelist some reference files that are referred to in comments.