Rewrite the `kill-buffer-delete-auto-save' tests (bug#59028)

Rewrite the `kill-buffer-delete-auto-save' tests (bug#59028)

These tests had been annoying to me when run in interactive mode.
They failed to bind `kill-buffer-delete-auto-save' as needed, so they
depended on the user's settings, and they failed to mock out the
"Buffer modified, save?" prompt.

* test/src/buffer-tests.el (test-kill-buffer-auto-save): New helper
function that mocks the two different kinds of prompts that occur when
killing modified buffers that visit a file.  Tests fail if expected
prompts are not issued.
(test-kill-buffer-auto-save-default): Use it, and explicitly bind
`kill-buffer-delete-auto-save' to nil.
(test-kill-buffer-auto-save-delete): Delete it.
(test-kill-buffer-auto-save-delete-yes): New test for the "yes" half
of the old `test-kill-buffer-auto-save-delete'.
(test-kill-buffer-auto-save-delete-yes): Ditto for the "no" half.