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/number/bitwise_and.wren

@@ -0,0 +1,12 @@
+IO.print(0 & 0) // expect: 0
+IO.print(2863311530 & 1431655765) // expect: 0
+IO.print(4042322160 & 1010580540) // expect: 808464432
+
+// Max u32 value.
+IO.print(4294967295 & 4294967295) // expect: 4294967295
+
+// Past max u32 value.
+IO.print(4294967296 & 4294967296) // expect: 0
+
+// TODO: Negative numbers.
+// TODO: Floating-point numbers.