Test non-UTF-8-encoded files that contain EF BB BF

Byte sequences that look like a UTF-8-encoded BOM must only be stripped from UTF-8-encoded files. This patch makes sure we actually test if stripping non-UTF-8 files is successfully prevented.

I changed the UTF-16 test files so that they start with `0xEF 0xBB 0xBF`.

```bash
$ hexdump -C fixture-utf16be
00000000  ef bb bf 00 00 55 00 6e  00 69 00 63 00 6f 00 72  |.....U.n.i.c.o.r|
00000010  00 6e 00 0a                                       |.n..|
00000014

$ hexdump -C fixture-utf16le
00000000  ef bb bf 00 55 00 6e 00  69 00 63 00 6f 00 72 00  |....U.n.i.c.o.r.|
00000010  6e 00 0a 00                                       |n...|
00000014
```