fix(engine): turn off the "emoticons_alt_exists" and "asciiart_alt_exists" rules

[shunguoy]

Rule bug: emoticons_alt_exists, asciiart_alt_exists

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

#805

Additional information can be found here:

Testing reference:

1. run the test/v2/checker/accessibility/rules/asciiart_alt_exists_ruleunit/AsciiArt-detectInvalid.html, the NR: "Verify that ASCII art has a text alternative" shouldn't show up.
2. run test/v2/checker/accessibility/rules/emoticons_alt_exists_ruleunit/Text-hasEmoticons.html, the NR: "Verify that ASCII art has a text alternative" shouldn't show up.

I have conducted the following for this PR:

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