Fix selection in test/visual.html

[jwmerrill]

In 51a75093dac047399e64a0d06a68c937c0df5a3e we accidentally started dispatching the CTRL-A keydown to the first textarea on the page instead of to the #selection-tests text field.

I wasn't previously familiar with the use of the second arg to the $() function, but it's a "context" in which selector will be applied. So I think

$('textarea', this)

is roughly equivalent to

$(this).find('textarea')

Ref:

• https://api.jquery.com/jQuery/#jQuery-selector-context

Reported in https://github.com/desmosinc/mathquill/issues/236

[jackschmidt]

This makes sense and fixes it for me! Thank you!