make sure vdiffr doesn't break with old versions of testthat

Author: thomasp85

tests/testthat/helper-vdiffr.R

@@ -2,7 +2,7 @@
 # VDIFFR_RUN_TESTS is explicitly set to "true", which should be the case only on
 # a GitHub Actions CI runner with stable version of R.
 
-if (requireNamespace("vdiffr", quietly = TRUE)) {
+if (requireNamespace("vdiffr", quietly = TRUE) && utils::packageVersion('testthat') >= '3.0.3') {
   expect_doppelganger <- vdiffr::expect_doppelganger
 } else {
   # If vdiffr is not available and visual tests are explicitly required, raise error.