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/implicit_receiver/static_methods.wren

@@ -0,0 +1,21 @@
+class Foo {
+  static getter {
+    IO.print("getter")
+  }
+
+  static setter=(value) {
+    IO.print("setter")
+  }
+
+  static method(a) {
+    IO.print("method")
+  }
+
+  static test {
+    getter            // expect: getter
+    setter = "value"  // expect: setter
+    method("arg")     // expect: method
+  }
+}
+
+Foo.test