Add a note about the skipped test to be reported.

inst/tests/tests.Rraw

@@ -18724,10 +18724,10 @@ if (test_bit64) local({
 })
 
 # non-ASCII plain symbol in by, #4708
+# NB: recall we can't use non-ASCII symbols in the test script. The text is a-<n-tilde>-o (year in Spanish)
 native_ano = iconv("a\U00F1o", "UTF-8", "")
 if (!is.na(native_ano)) { # #6339: symbol must be represented in native encoding
   DT = data.table(a = rep(1:3, 2))
-  # NB: recall we can't use non-ASCII symbols here. the text is a-<n-tilde>-o (year in Spanish)
   setnames(DT, "a", native_ano)
   test(2266, eval(parse(text=sprintf("DT[ , .N, %s]$N[1L]", native_ano))), 2L)
   # sub-key can also be retained in plain query, part of #4498
@@ -18736,6 +18736,8 @@ if (!is.na(native_ano)) { # #6339: symbol must be represented in native encoding
   test(2266.2, key(DT[ , .(grp)]), NULL)
   ## renaming also caught
   test(2266.3, key(DT[ , .(newid = id, newgrp = grp)]), c('newid', 'newgrp'))
+} else {
+  cat("Tests 2266* skipped. n-tilde cannot be represented in the native encoding on this system. Please report\n")
 }
 
 # all.equal failed to dispatch to methods of columns, #4543