fix(engine): fix issues related to element text calculation

[shunguoy]

• Rule bug: label_content_exists, input_label_before

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

#1557

Additional information can be found here:

Testing reference:

test case: test/v2/checker/accessibility/rules/label_content_exists_ruleunit/label-invisible-element.html
The following false positive violations should be fixed:

• The element does not have descriptive text that identifies the expected input
• Form control element has no associated label

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.