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/core/string/subscript.wren

@@ -0,0 +1,42 @@
+// Returns characters (as strings).
+IO.print("abcd"[0]) // expect: a
+IO.print("abcd"[1]) // expect: b
+IO.print("abcd"[2]) // expect: c
+IO.print("abcd"[3]) // expect: d
+
+// Allows indexing backwards from the end.
+IO.print("abcd"[-4]) // expect: a
+IO.print("abcd"[-3]) // expect: b
+IO.print("abcd"[-2]) // expect: c
+IO.print("abcd"[-1]) // expect: d
+
+// Regression: Make sure the string's internal buffer size is correct.
+IO.print("abcd"[1] == "b") // expect: true
+
+// Indexes by byte, not code point.
+//
+// Bytes:           11111
+//        012345678901234
+// Chars: sø mé ஃ  thî ng
+IO.print("søméஃthîng"[0]) // expect: s
+IO.print("søméஃthîng"[1]) // expect: ø
+IO.print("søméஃthîng"[3]) // expect: m
+IO.print("søméஃthîng"[6]) // expect: ஃ
+IO.print("søméஃthîng"[10]) // expect: h
+IO.print("søméஃthîng"[-1]) // expect: g
+IO.print("søméஃthîng"[-2]) // expect: n
+IO.print("søméஃthîng"[-4]) // expect: î
+
+// If the subscript is in the middle of a UTF-8 sequence, yield an empty string.
+IO.print("søméஃthîng"[2] == "") // expect: true
+IO.print("søméஃthîng"[7] == "") // expect: true
+IO.print("søméஃthîng"[8] == "") // expect: true
+IO.print("søméஃ"[-1] == "") // expect: true
+IO.print("søméஃ"[-2] == "") // expect: true
+
+// 8-bit clean.
+IO.print("a\0b\0c"[0] == "a") // expect: true
+IO.print("a\0b\0c"[1] == "\0") // expect: true
+IO.print("a\0b\0c"[2] == "b") // expect: true
+IO.print("a\0b\0c"[3] == "\0") // expect: true
+IO.print("a\0b\0c"[4] == "c") // expect: true