Need whitespace trimming consistency for computedlabel in WPT tests.

I recalled that AccName trimmed whitespace in each step and once again at the end. However in this WPT review comment, @spectranaut noted that it either never did, or has been changed to remove those requirements. I started digging and noticed there were many outstanding whitespace and trimming related issues in the AccName WhiteSpace Project.

Until there is a required whitespace consistency in AccName, we've resolved to trim the whitespace before comparison in WPT, normalizing the testing result even if the implementations have minor differences. If there is a resolution to address trimming again in AccName, please file a related issue to cull this workaround from /wai-aria/scripts/aria-util.js in the WPT source. Thanks.