Add test for rendering markdown in report previewer

[asbates]

Markdown rendering in report previewer was added in #155. However there were no tests added for this because it would require upgrading testthat. So if and when we upgrade to testthat 3rd edition, add a test to ensure markdown renders correctly.

testthat::test_that("render_text_block_preview - markdown renders to html fragment", {

  block <- TextBlock$new()

  block$set_content(
    "
  **bold text**
  - item 1
  - item 2
  "
  )

  testthat::expect_snapshot(render_text_block_preview(block$get_content()))
})

[chlebowa]

Acceptance criteria:
Add unit tests for rendering reports.