(v1.4.0.9053) unit test old R versions

tests/testthat.R

@@ -23,7 +23,11 @@
 # how to conduct AMR analysis: https://msberends.github.io/AMR/        #
 # ==================================================================== #
 
-library(testthat, warn.conflicts = FALSE)
-library(AMR)
-
-test_check("AMR")
+if (require("testthat")) {
+  # the testthat package is in Suggests, but very old R versions will not be
+  # able to install it. Yet, we want checks in those R versions as well, so
+  # only run unit tests in later R versions:
+  library(testthat, warn.conflicts = FALSE)
+  library(AMR)
+  test_check("AMR")
+}