whitespace alt in srcset tests

[cookiecrook]

Additional suggestion from @giacomo-petri that didn't land in the merge of web-platform-tests/wpt#52480

<!-- replicating all former generic test cases that used empty alt text, now also using whitespace-only alt text, per https://github.com/w3c/aria/pull/2706 -->

<img data-testname="el-img-whitespace-alt-aria-label-empty" class="ex-generic" srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" alt=" " aria-label="">
<img data-testname="el-img-whitespace-alt-aria-label-whitespace" class="ex-generic" srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" alt=" " aria-label=" ">
<img data-testname="el-img-whitespace-alt-aria-labelledby-non-existing" class="ex-generic" srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" alt=" " aria-labelledby="non-existing"> 
<img data-testname="el-img-whitespace-alt-aria-labelledby-empty" class="ex-generic" srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" alt=" " aria-labelledby="empty">
<img data-testname="el-img-whitespace-alt-aria-labelledby-whitespace" class="ex-generic" srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" alt=" " aria-labelledby="space">
<img data-testname="el-img-whitespace-alt-title" class="ex-generic" srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" alt=" " title="x">
<img data-testname="el-img-whitespace-alt-title-empty" class="ex-generic" srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" alt=" " title="">
<img data-testname="el-img-whitespace-alt-title-whitespace" class="ex-generic" srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" alt=" " title=" ">
<img data-testname="el-img-whitespace-alt-no-src-or-srcset" class="ex-generic" alt=" ">
<img data-testname="el-img-whitespace-alt-no-src-srcset-title-empty" class="ex-generic" srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" alt=" " title="">
<img data-testname="el-img-whitespace-alt-no-src-srcset-title-whitespace" class="ex-generic" srcset="data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACH5BAEAAAAALAAAAAABAAEAAAICRAEAOw==" alt=" " title=" ">
<img data-testname="el-img-whitespace-alt-srcset-invalid-value" class="ex-generic" srcset="..." alt=" ">

As briefly discussed in w3c/html-aam#605, adding whitespace only testcases

Originally posted by @giacomo-petri in web-platform-tests/wpt#52480 (comment)