expect_snapshot for UI functions

Expected result is stored in a separate snapshot file

• If we can also test

test_that("sliderInput01() creates expected HTML", {
  expect_equal(as.character(sliderInput01("x")), "<div class=\"form-group shiny-input-container\">\n  <label class=\"control-label\" id=\"x-label\" for=\"x\">x</label>\n  <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\"/>\n</div>")
})
#> Test passed 😀

But it makes too hard to see if the output changes