gh-116608: Ignore UTF-16 BOM in importlib.resources._functional tests (GH-117569)

gh-116609: Ignore UTF-16 BOM in importlib.resources._functional tests

To test the `errors` argument, we read a UTF-16 file as UTF-8
with "backslashreplace" error handling. However, the utf-16
codec adds an endian-specific byte-order mark, so on big-endian
machines the expectation doesn't match the test file (which was
saved on a little-endian machine).

Use endswith to ignore the BOM.