Gandi provider: refactor tests to be more readable, robusts, and extensible; and minor fixes

[gfaugere]

Description

This refactors the gandi provider tests to make them more robust, easier to extend, and hopefully more readable

This also includes two minor fixes :

• the conversion of an endpoint DNSName to @ for apex domains is not robust (as pointed out here - a test case was added to cover this)
• the TTL field is not filled when fetching endpoints (discovered when rewriting the tests)

This relates to #3855 - I am trying to make changes to the Gandi provider more reliable and reduce noise on improvement/feature PRs

Checklist

• Unit tests updated
• End user documentation updated (no change in functionality, doc was not touched)

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[k8s-ci-robot]

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[mloiseleur]

/ok-to-test

[mloiseleur]

I can confirm that c5b488d and 5a6e12f are covered by the test.
Thanks @gfaugere !
/lgtm

[mloiseleur]

cc @johngmyers

[gfaugere]

Hey guys, any interest in this PR? Anything I can do to help get it merged?

[johngmyers]

/approve

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: johngmyers

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[johngmyers]

/retest