Add initial shinytest to verify the quosure support for shiny's expression functions

[MadhulikaTanuboddi]

More details and connected to https://github.com/rstudio/shiny/pull/3472 https://trello.com/c/Iuf1XAlW/2500-add-shinytest-tests-for-recent-quosure-fixes-updates

TODO:

• [x] Add more lowel level functionality like reactive, observe, and observeEvent to the app and the quosure support/usage
• [x] Update shinytest accordingly once above is done
• [x] Verify GHA builds are passing

cc: @shalutiwari

[schloerke]

Green checks in https://github.com/rstudio/shinycoreci-apps/runs/3264926312

Merging