(v1.6.0.9024) unit test update

tests/testthat.R

@@ -27,6 +27,7 @@
 # able to install it. Yet, we want basic R CMD CHECK's in those R versions 
 # as well, so only run unit tests in later R versions:
 if (require("testthat", warn.conflicts = FALSE)) {
+  library(testthat)
   library(AMR)
   test_check("AMR")
 }