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/core/object/not.wren

@@ -0,0 +1,2 @@
+class Foo {}
+IO.print(!(new Foo)) // expect: false

test/core/object/to_string.wren

@@ -0,0 +1,2 @@
+class Foo {}
+IO.print((new Foo).toString == "instance of Foo") // expect: true