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/field/nested_class.wren

@@ -0,0 +1,18 @@
+class Outer {
+  method {
+    _field = "outer"
+    IO.print(_field) // expect: outer
+
+    class Inner {
+      method {
+        _field = "inner"
+        IO.print(_field) // expect: inner
+      }
+    }
+
+    (new Inner).method
+    IO.print(_field) // expect: outer
+  }
+}
+
+(new Outer).method