:enc(utf16) assumes host byte order if there is no BOM, so test accordingly.
:enc(utf16) assumes host byte order if there is no BOM, so test accordingly.

The test had always used 'sample-UTF-16LE.txt' when testing :enc(utf16), but
that is only appropriate on little endian systems. For big endian systems,
we need to test with 'sample-UTF-16BE.txt' instead.

Fixes #693

perl6/roast,dogbert17/roast,dogbert17/roast