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/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