UTF8 BOM Seems to be Missing from UTF8BOM Test

[InsertCreativityHere]

We have a test to ensure that the Slice compilers can correctly tolerate the presence of Byte-Order-Markers in Slice files,
https://github.com/zeroc-ice/ice/blob/main/cpp/test/Slice/utf8BOM/Test.ice

But, opening this file up in a hex-editor, there is no BOM at the beginning of the file.
I guess it was deleted by an auto-formatter or one of the times we changed the copyright headers.

We should add the BOM back, and change the test so it fails if the BOM was deleted somehow.