Fix CI by skipping visual tests when vdiffr is not available (#4192)

* Skip visual tests when vdiffr is not available

* Add tweak to skip ggsave tests

* Update CMD-ckeck.yml

* Add http-user-agent

* Fix a comment

* Require vdiffr except when it's explicitly disabled

Author: yutannihilation

tests/testthat/helper-vdiffr.R

@@ -2,4 +2,14 @@
 #   - VDIFFR_RUN_TESTS is "false" (on Travis CI with older versions and dev version of R)
 #   - CI is not set (on CRAN)
 
-expect_doppelganger <- vdiffr::expect_doppelganger
+if (requireNamespace("vdiffr", quietly = TRUE)) {
+  expect_doppelganger <- vdiffr::expect_doppelganger
+} else {
+  # If vdiffr is not available and visual tests are not explicitly disabled, raise error.
+  if (!identical(Sys.getenv("VDIFFR_RUN_TESTS"), "false")) {
+    abort("vdiffr is not installed")
+  }
+
+  # Otherwise, assign a dummy function
+  expect_doppelganger <- function(...) skip("vdiffr is not installed.")
+}