[Backport 3.6] Fix compiler warnings in test_suite_pk.function

[eleuzi01]

Description

Trivial 3.6 backport of #9250

PR checklist

Please tick as appropriate and edit the reasons (e.g.: "backport: not needed because this is a new feature")

• changelog not required
• 3.6 backport not required
• 2.28 backport not required
• tests not required

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[tom-cosgrove-arm]

The Changelog entry should be included in the backport - and note that in the original PR it is missing a newline on the last line, even though GitHub doesn't give its usual warning icon there (maybe we should check for this explicitly in check_files, since it happens reasonably often?)

[gilles-peskine-arm]

missing a newline on the last line, even though GitHub doesn't give its usual warning icon there (maybe we should check for this explicitly in check_files

We do.

[tom-cosgrove-arm]

LGTM - faithful backport

[minosgalanakis]

LGTM

[mpg]

@eleuzi01 Gentle reminder to apply relevant labels to your backport PRs as well :) (Noticed during triage.)