tests: improve compat with macOS

* tests/misc/wc-nbsp.sh: Only the en_US.iso8859-1 form
is accepted on macOS 10.15.7 at least.  GNU/Linux also
accepts ISO-8859-1 (and canonicalizes the charmap to this).

tests/misc/wc-nbsp.sh

@@ -31,8 +31,8 @@ check_word_sep() {
   fi
 }
 
-export LC_ALL=en_US.ISO-8859-1
-if test "$(locale charmap 2>/dev/null)" = ISO-8859-1; then
+export LC_ALL=en_US.iso8859-1  # only lowercase form works on macOS 10.15.7
+if test "$(locale charmap 2>/dev/null | sed 's/iso/ISO-/')" = ISO-8859-1; then
   check_word_sep '\xA0'
 fi