Fix and unfudge incorrect Pair binding test

The current version attempts to bind a Str variable to a Signature
with one named arg in it. The test is fudged, as :$a gets a Mu
value instead of $b's value, and it doesn't outright die only
because Str.Capture returns an empty capture.

Fix by binding a proper Capture, with $b passed as a named :a arg.

S06-other/pairs-as-lvalues.t

@@ -11,7 +11,7 @@ throws-like 'my $var; (key => $var) = "value"', X::Assignment::RO;
 {
     my ($a, $b);
     $b = 'b';
-    :(:$a) := $b;
+    :(:$a) := \(:a($b));
     is $a, 'b', 'can bind to single pair';
     ok $a =:= $b, 'variables are bound together (?)';
 }