fixrule(element_scrollable_tabbable, label_name_visible): fix issues related to off-screen and scrollable content

[shunguoy]

• Rule bug: element_scrollable_tabbable, label_name_visible

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

Fixes #1911
Fixes #1889

Testing reference:

-for #1911: test/v2/checker/accessibility/rules/element_scrollable_tabbable_ruleunit/textarea_pass.html
The fixed violation: "The scrollable element <textarea> with non-interactive content is not tabbable"
-for #1889: test/v2/checker/accessibility/rules/label_name_visible_ruleunit/label_offscreen.html
One of the two violations is fixed (another is a correct reporting): "Accessible name does not match or contain the visible label text "

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.

[ErickRenteria]

Validated