update CHANGELOG_PENDING tests to work with the new `make changelog` command.

[dixler]

Hello!

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Issue details

We have updated the way that we do changelogs, but our repository has tests that don't reflect the new workflow. We should update our changelog test to detect that a file has been added to changelog/pending/ on commits to pass the test.

Affected area/feature

[dixler]

cc @AaronFriel