fix(form-radio/form-checkbox): ensure prop required propagated in group mode (fixes #2839)

[tmorehouse]

Description of Pull Request:

form-radios and form-checkboxes were not rendering with required attribute when prop required was set on the group. The original test was checking for the wrong attribute, and has now been corrected as well.

Fixes #2839

PR checklist:

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Enhancement to an existing feature
• ARIA accessibility
• Documentation update
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact:

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (i.e.
  fixes #xxxx[,#xxxx], where "xxxx" is the issue number)
• The PR should address only one issue or feature. If adding multiple features or fixing a bug
  and adding a new feature, break them into separate PRs if at all possible.
• PR titles should following the
  Conventional Commits naming convention (i.e.
  "fix(alert): not alerting during SSR render", "docs(badge): Updated pill examples, fix typos",
  "chore: fix typo in docs", etc). This is very important, as the CHANGELOG is generated
  from these messages.

If new features/enhancement/fixes are added or changed:

• Includes documentation updates (including updating the component's package.json for slot and
  event changes)
• New/updated tests are included and passing (if required)
• Existing test suites are passing
• The changes have not impacted the functionality of other components or directives
• ARIA Accessibility has been taken into consideration (does it affect screen reader users or
  keyboard only users? clickable items should be in the tab index, etc)

If adding a new feature, or changing the functionality of an existing feature, the PR's
description above includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a
  suggestion issue first and wait for approval before working on it)

[codecov]

Codecov Report

Merging #2842 into dev will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##              dev    #2842   +/-   ##
=======================================
  Coverage   83.97%   83.97%           
=======================================
  Files         196      196           
  Lines        3183     3183           
  Branches      932      933    +1     
=======================================
  Hits         2673     2673           
  Misses        371      371           
  Partials      139      139

Impacted Files	Coverage Δ
src/mixins/form-radio-check.js	100% <100%> (ø)	⬆️

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