Add advice to do test config somewhere other than tests/testhat.R (#1635

)

Closes #1631

inst/templates/testthat.R

@@ -1,3 +1,11 @@
+# This file is part of the standard setup for testthat.
+# It is recommended that you do not modify it.
+#
+# Where should you do additional test configuration?
+# Learn more about the roles of various files in:
+# * https://r-pkgs.org/tests.html
+# * https://testthat.r-lib.org/reference/test_package.html#special-files
+
 library(testthat)
 library({{{ name }}})