Update shinytest2 screenshot defaults to avoid relayout issues

[gadenbuie]

Following up on rstudio/chromote#129, there are two things that shinytest2 can do:

1. When selector = "viewport", shinytest2 can set captureBeyondViewport = FALSE for chromote's screenshot method.

2. When selector = "scrollable_area", shinytest2 could query Chrome to determine how big the scrollable area is, then resize the viewport to match that size before screenshotting with captureBeyondViewport = FALSE, followed by restoring the viewport size.

   This option is more or less the pattern used by puppeteer, mentioned in ChromoteSession$screenshot() triggers a page resize before screenshot. chromote#96 (comment).