Add basic tests for ActionDispatch::ExceptionWrapper
[CI SKIP] Corrections to testing guide.

Partially revert https://github.com/rails/rails/commit/c31e638a37946a120f6dbf207127d36beea5ab85.
Merge pull request #17619 from tgxworld/fixes_to_testing_guide

[ci skip] Corrections to testing guide.
Merge pull request #17614 from gsamokovarov/exception-wrapper-basic-tests

Add basic tests for ActionDispatch::ExceptionWrapper