refactor: move benchmark directory under test and update all references

The benchmark directory is relocated from the project root into the test directory, and all scripts, documentation links, and gitignore paths are updated accordingly. The test runner is also refactored to explicitly walk subdirectories (core, io, language, limit) instead of using a single test root, and a new test/README.md is added to document the test suite structure. Additionally, the constructor test for the Foo class is updated to add a zero-arity constructor and adjust expected output strings.

test/language/closure/close_over_later_variable.wren

@@ -0,0 +1,13 @@
+// This is a regression test. There was a bug where if an upvalue for an
+// earlier local (here "a") was captured *after* a later one ("b"), then Wren
+// would crash because it walked to the end of the upvalue list (correct), but
+// then didn't handle not finding the variable.
+
+new Fn {
+  var a = "a"
+  var b = "b"
+  new Fn {
+    IO.print(b) // expect: b
+    IO.print(a) // expect: a
+  }.call()
+}.call()