* yasnippet-tests.el (yas-mock-insert): Use `ert-simulate-command' to
simulate n self-insert-commands.
(simple-mirror, mirror-with-transformation):
(primary-field-transformation, nested-placeholders-kill-superfield):
(nested-placeholders-use-subfield):
(mirrors-adjacent-to-fields-with-nested-mirrors):
(deep-nested-mirroring-issue-351, example-for-issue-271):
(string-match-with-subregexp-in-mirror-transformations): Call
`yas-mock-insert' directly instead of with `ert-simulate-command'.