Increase test coverage in csv module.

[JanEricNitschke]

The csv (python) module currently has 10 untested lines:

Name         Stmts   Miss Branch BrPart  Cover   Missing
--------------------------------------------------------
Lib/csv.py     262     10     96      8    95%   107->109, 164-165, 171, 255, 305->307, 312->314, 316-317, 327, 431, 434->443, 474, 511
--------------------------------------------------

Five of these are fairly easily testable. Specifically the Sniffer no delimiter error, DictReader fielname setter and case where fieldname determination from the first row fails because the iterator is exhausted.

Linked PRs

• gh-138621: Increase test coverage for csv.DictReader and csv.Sniffer #138622