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/this/nested_class.wren

@@ -0,0 +1,22 @@
+class Outer {
+  method {
+    IO.print(this) // expect: Outer
+
+    new Fn {
+      IO.print(this) // expect: Outer
+
+      class Inner {
+        method {
+          IO.print(this) // expect: Inner
+        }
+        toString { "Inner" }
+      }
+
+      (new Inner).method
+    }.call()
+  }
+
+  toString { "Outer" }
+}
+
+(new Outer).method