Use double-tilde for strikethrough in Markdown tests (#4144)

* Use double-tilde for strikethrough in Markdown tests

The current dev version of commonmark has disabled single tilde   for strikethrough: r-lib/commonmark#33 Using double-tilde will make it consistent with Pandoc's Markdown (where single-tilde is for subscripts).

* Skip tests if commonmark is outdated

* Revert "Skip tests if commonmark is outdated"

This reverts commit 97bee20.

---------

Co-authored-by: Carson Sievert <cpsievert1@gmail.com>

tests/testthat/test-inline-markdown.R

@@ -11,18 +11,18 @@ test_that("HTML has correct attributes", {
 })
 
 test_that("Github extensions are on by default", {
-  html <- markdown("a ~paragraph~ with a link: https://example.com")
+  html <- markdown("a ~~paragraph~~ with a link: https://example.com")
   expect_equal(html, HTML("<p>a <del>paragraph</del> with a link: <a href=\"https://example.com\">https://example.com</a></p>\n"))
 })
 
 test_that("Github extensions can be disabled", {
-  html <- markdown("a ~paragraph~", extensions = FALSE)
-  expect_equal(html, HTML("<p>a ~paragraph~</p>\n"))
+  html <- markdown("a ~~paragraph~~", extensions = FALSE)
+  expect_equal(html, HTML("<p>a ~~paragraph~~</p>\n"))
 })
 
 test_that("Additional options are respected", {
-  html <- markdown("a ~paragraph~", extensions = FALSE, sourcepos = TRUE)
-  expect_equal(html, HTML("<p data-sourcepos=\"1:1-1:13\">a ~paragraph~</p>\n"))
+  html <- markdown("a ~~paragraph~~", extensions = FALSE, sourcepos = TRUE)
+  expect_equal(html, HTML("<p data-sourcepos=\"1:1-1:15\">a ~~paragraph~~</p>\n"))
 })
 
 test_that("Multiline markdown works properly", {