chore: Improve usage of test_name in the Acceptance test workflow

[maastha]

Description

This PR improves usage of test_name in the Acceptance test workflow .

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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

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Further comments

[lantoli]

nice!

I have done a few executions to double-check the 3 conditions in ACCTEST_REGEX_RUN:

• using config package so it executes all mig & acc tests for that package: https://github.com/mongodb/terraform-provider-mongodbatlas/actions/runs/9569174218
• also using a test_name TestAccConfigRSAlertConfiguration_basic, it only executes it: https://github.com/mongodb/terraform-provider-mongodbatlas/actions/runs/9569205511
• using config and a provider version 1.17.1 it only executes mig tests for config: https://github.com/mongodb/terraform-provider-mongodbatlas/actions/runs/9569267701

[lantoli]

knit: these two lines can be reverted:

mustTrigger: ${{ github.event_name == 'schedule' || (github.event_name == 'workflow_dispatch' && inputs.test_group == '' && inputs.test_name == '')  }}

if: ${{ inputs.test_group == '' && inputs.test_name == '' && env.mustTrigger == 'false' }}

to the previous version without test_name:

mustTrigger: ${{ github.event_name == 'schedule' || (github.event_name == 'workflow_dispatch' && inputs.test_group == '' )  }}

if: ${{ inputs.test_group == ''  && env.mustTrigger == 'false' }}

We're saying that when using test_name a test_group must be specified, so if we're not using a test_group ( inputs.test_group == '') then we're sure that test_name is empty, no need to check it

[EspenAlbert]

Nice!