tests: Detect session encoding incapable of U+00F1 (#6567)

iconv() does not necessarily fail to convert U+00F1 to ASCII. For
example, FreeBSD iconv() succeeds and returns '?' instead of the
character in question. Use identical() to compare the result of the
conversion back to the original (which internally converts both to
UTF-8).

inst/tests/tests.Rraw

@@ -18768,7 +18768,7 @@ 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
+if (identical(native_ano, "a\U00F1o")) { # #6339: symbol must be represented in native encoding
   DT = data.table(a = rep(1:3, 2))
   setnames(DT, "a", native_ano)
   test(2266, eval(parse(text=sprintf("DT[ , .N, %s]$N[1L]", native_ano))), 2L)