fixrule(aria_hidden_nontabbable) improve group message

[philljenkins]

Rule/Engine updates:

fixrule(ruleid): improve group msg in rule and baselines

This PR is related to the following issue(s):

• fixrule(aria_hidden_nontabbable) improve group mesage #1395

Additional information can be found here:

• ruleID was changed to aria_hidden_nontabbable during ruleID consistency work:
  message:
  "group": "A focusable element should not be within the subtree of an element with 'aria-hidden' set to \"true\"",
  changed to:
  "group": "A hidden element should not contain any tabbable elements",
• "group" message is displayed in Help, not in test files, so no changes are needed in test cases
• "group" message changed in rule and baselines files

Testing reference:

• regular build testing
  test/v2/checker/accessibility/rules/aria_hidden_notabbale_ruleunit/*

I have conducted the following for this PR:

• I validated this code in Chrome and FF
• I validated this fix in my local env
• I provided details for testing
• This PR has been reviewed and is ready for test
• I understand that the title of this PR will be used for the next release notes.

[philljenkins]

@shunguoy Why shouldn't the baseline files be checked in?
such as ...accessibility-checker/test/baselines/baslines.verify.toolkit.json

26c8641