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/iterate.wren

@@ -0,0 +1,24 @@
+var s = "abçd"
+IO.print(s.iterate(null)) // expect: 0
+IO.print(s.iterate(0)) // expect: 1
+IO.print(s.iterate(1)) // expect: 2
+// Skip 3 because that's the middle of the ç sequence.
+IO.print(s.iterate(2)) // expect: 4
+// Iterating from the middle of a UTF-8 sequence goes to the next one.
+IO.print(s.iterate(3)) // expect: 4
+IO.print(s.iterate(4)) // expect: false
+
+// Out of bounds.
+IO.print(s.iterate(123)) // expect: false
+IO.print(s.iterate(-1)) // expect: false
+
+// Nothing to iterate in an empty string.
+IO.print("".iterate(null)) // expect: false
+
+// 8-bit clean.
+IO.print("a\0b\0c".iterate(null)) // expect: 0
+IO.print("a\0b\0c".iterate(0)) // expect: 1
+IO.print("a\0b\0c".iterate(1)) // expect: 2
+IO.print("a\0b\0c".iterate(2)) // expect: 3
+IO.print("a\0b\0c".iterate(3)) // expect: 4
+IO.print("a\0b\0c".iterate(4)) // expect: false