Send \n and end of chars echo server message.

The \n serves as a normalization terminator - that is, it means we
know that even if a combining char comes next, it will end up as an
isolate rather than forming a grapheme with the \n. A `z` could be
followed by a combiner in the next packet, so after upcoming fixes
to async socket decoding we'll now not hand such a character on
until the next packet arrives and we see there is no combiner.
This would break the test, which did not account for such NFG
behavior.

Author: jnthn

S32-io/IO-Socket-Async.t

@@ -66,7 +66,7 @@ multi sub client(Str $message) {
     });
 }
 
-my $message = [~] '0'..'z';
+my $message = [~] flat '0'..'z', "\n";
 my $echoResult = await client($message);
 $echoTap.close;
 ok $echoResult eq $message, 'Echo server';