expect_snapshot for UI functions

Expected result is stored in a separate snapshot file

• If we have

sliderInput01 <- function(id) {
  sliderInput(id, label = id, min = 0, max = 1, value = 0.5, step = 0.1)
}

cat(as.character(sliderInput01("x")))
#> <div class="form-group shiny-input-container">
#>   <label class="control-label" id="x-label" for="x">x</label>
#>   <input class="js-range-slider" id="x" data-skin="shiny" data-min="0" data-max="1" data-from="0.5" data-step="0.1" data-grid="true" data-grid-num="10" data-grid-snap="false" data-prettify-separator="," data-prettify-enabled="true" data-keyboard="true" data-data-type="number"/>
#> </div>