chore: Enables and refactors TestAccBackupRSCloudBackupSchedule_export test.

[marcosuma]

Description

This test was disabled from CI because it needed AWS configuration. Since this is not really a blocker, I am proposing some changes and activate it.

Type of change:

• [ ] Bug fix (non-breaking change which fixes an issue). Please, add the "bug" label to the PR.
• [ ] New feature (non-breaking change which adds functionality). Please, add the "enhancement" label to the PR. A migration guide must be created or updated if the new feature will go in a major version.
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected). Please, add the "breaking change" label to the PR. A migration guide must be created or updated.
• [ ] This change requires a documentation update
• [ ] Documentation fix/enhancement

Required Checklist:

• [x] I have signed the MongoDB CLA
• [x] I have read the contributing guides
• [x] I have checked that this change does not generate any credentials and that they are NOT accidentally logged anywhere.
• [x] I have added tests that prove my fix is effective or that my feature works per HashiCorp requirements
• [x] I have added any necessary documentation (if appropriate)
• [x] I have run make fmt and formatted my code
• [x] If changes include deprecations or removals, I defined an isolated PR with a relevant title as it will be used in the auto-generated changelog.
• [x] If changes include removal or addition of 3rd party GitHub actions, I updated our internal document. Reach out to the APIx Integration slack channel to get access to the internal document.

Further comments

[marcosuma]

@EspenAlbert

Maybe we can create an acc helper for the boilerplate resources?

I tend to prefer that the test definition should all be together, verbosity and repetition in test is not a bad thing, actually the opposite because it makes the understanding process way faster.

Said that, I think it could be a future refactoring, for now let's just make sure we can rely on these tests.

Note also I had to make some further changes because the test was actually broken and wasn't easily to figure because of some unrelated flakyness.

[marcosuma]

Unrelated test TestAccBackupRSOnlineArchiveWithProcessRegion is failing, overall backup test suite is being flaky in the last few days. Going ahead with merging this.