gh-138621: Increase test coverage for csv.DictReader and csv.Sniffer (GH-138622)

* Increase test coverage for csv.DictReader and csv.Sniffer

Previously there were no tests for the DictReader fieldnames
setter, the case where a StopIteration was encountered when trying
to determine the fieldnames from the content or the case where
Sniffer could not find a delimiter.

* Revert whitespace change to comment

* Add a test that csv.Sniffer.has_header checks up to 20 rows

* Replace name and age with letter and offset

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

* Address review comment

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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>