Test synthetic numerals in colon pairs in :42foo format throw

RT#128551: https://rt.perl.org/Ticket/Display.html?id=128551

Commit with fix: rakudo/rakudo@4663d43130

S02-literals/pairs.t

@@ -23,7 +23,7 @@ use Test;
 #   S02 lists ':a' as being equivlaent to a => 1, so
 #   the type of the value of that pair is Int, not Bool
 
-plan 76;
+plan 77;
 
 sub f1n (:$a) { $a.WHAT.gist }
 sub f1p ( $a) { $a.WHAT.gist }
@@ -175,4 +175,8 @@ sub f9 (:$bar!) { WHAT($bar) }
     }
 }
 
+# RT #128821
+throws-like ":7\x[308]a", X::Syntax::Malformed,
+    'synthetic numerals in colon pairs in :42foo format throw';
+
 # vim: ft=perl6