Detect use of `testthat` in `DESCRIPTION`

[etiennebacher]

So that --extend-select TESTTHAT isn't necessary in this case

[etiennebacher]

I'm actually not sure this is useful:

• if you want to have those rules active all the time, just make a jarl.toml with extend-select = ["TESTTHAT"] so that others also see this;
• if you want to run this only once, just use --extend-select TESTTHAT

[etiennebacher]

This should be part of the broader discussion in #337