Reorganize tests and benchmark scripts.

Mainly to get rid of one top level directory. But this will
also be useful when there are tests of the embedding API.

test/language/setter/associativity.wren

@@ -0,0 +1,18 @@
+class Foo {
+  new(value) { _value = value }
+  toString { _value }
+  bar=(value) {
+    _value = value
+    return value
+  }
+}
+
+var a = new Foo("a")
+var b = new Foo("b")
+var c = new Foo("c")
+
+// Assignment is right-associative.
+a.bar = b.bar = c.bar = "d"
+IO.print(a.toString) // expect: d
+IO.print(b.toString) // expect: d
+IO.print(c.toString) // expect: d