Add multi-method tests from nqpclr, which now pass in nqp-rx/nom.

t/nqp/55-multi-method.t

@@ -0,0 +1,40 @@
+#! nqp
+
+plan(4);
+
+class Foo {
+    proto method bar($x?) { * }
+    multi method bar() { 42 }
+    multi method bar($x) { 2 * $x }
+}
+
+my $x := Foo.new();
+
+if $x.bar() == 42 {
+    say("ok 1");
+}
+
+if $x.bar(5) == 10 {
+    say("ok 2");
+}
+
+class Baz is Foo {
+    multi method bar() { 37 }
+}
+
+my $y := Baz.new();
+
+if $y.bar() == 37 {
+    say("ok 3");
+}
+
+class Quux is Foo {
+    proto method bar() { * }
+    multi method bar() { 37 }
+}
+
+my $z := Quux.new();
+
+if $z.bar() == 37 {
+    say("ok 4");
+}