fix(Tabs): correct aria-setsize count when Tabs are conditionally rendered

[nathanyoung]

Github Issue or Trello Card

This PR addresses this issue: #805

Counting react children: https://www.agney.dev/blog/react-children-count

What type of change is this?

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Updating documentation.
• Updating deployment/build pipeline.
• Updating Project Dependencies
• Improving or adding to Test Coverage

Completeness Checklist

• TESTS: My changes maintain the baseline required test coverage, as specified by code climate analysis.
• DOCS: All new component work is covered in that component's Overview docs.
• DOCS: All new component work is covered in a component Playground.
• DOCS: All new visual changes or options are covered under relevant components' VisualTests.
• My code has no linting or typescript compile warnings.
• My work is tied to a Github issue and satisfies the acceptance criteria (if applicable) of the corresponding issue.

Quality Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

UI Checklist

• I have conducted visual UAT on my changes/features.
• My solution works well on desktop, tablet, and mobile browsers.

[codeclimate]

Code Climate has analyzed commit 1c20ff4 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (95% is the threshold).

This pull request will bring the total coverage in the repository to 97.9%.

View more on Code Climate.