tests: RuboCop checks .docs/CustomTags.rb

Bump RuboCop to 0.27.1 so it can recognize '.docs', see upstream bug for
details: https://github.com/bbatsov/rubocop/issues/1401

Fix minor whitespaces problem that `rubocop --auto-gen-config` did not
put to .rubocop_todo.yml file.

Bug: 63307
Change-Id: I0a640135ca8e63a8efd6e8dc0e982a05097c20aa