add textgen test facility

plaintextgen can be tested by generator tests, but it could make sense to provide textgen test facility for people who want to compare files (e.g. when using the textgen aspect, or want to look at encoding of a file)

Start by locating the existing plaintextgen generator tests and the textgen aspect mentioned in the issue. Determine how a test facility could support file comparison and inspection of file encoding; the issue does not name specific files or define an implementation boundary.

Written by the indexing model from the issue text.