Test :sym is reserved on subs in 6.d

Per 6.d-prep TODOs
POV: rakudo/rakudo@a68784a6f6

MISC/misc-6.d.t

@@ -0,0 +1,12 @@
+use v6.d.PREVIEW;
+use Test;
+
+# This file is for various 6.d-specific tests that require the use of `v6.d.PREVIEW` pragma.
+# They might be rearranged into other files in the future, but for now, keeping them in this
+# file avoids creating a ton of `-6.d.t` files for the sake of single tests.
+
+plan 1;
+throws-like ｢use v6.d.PREVIEW; sub foo:sym<z> {}｣, X::Syntax::Reserved,
+    ':sym<> token is reserved on subs in 6.d';
+
+# vim: expandtab shiftwidth=4 ft=perl6