atest: Renaming test data files to use .robot extension (#1766).

Another massive commit. Now all .txt have been converted to .robot
except for files that actually contain just text or that are test data
for testing .txt format. Remaining task is converting .html (and
possible .tsv) files to .robot (except obviously for files related to
testin .html/tsv).

This is quite a bit of work. Hopefully it will pay back later...