Fix encoding with 'utf-8-auto'

Fix encoding with 'utf-8-auto'

* src/coding.c (encode_coding_utf_8): Fix encoding with
'utf-8-auto': it should produce BOM, per the documentation of
the :bom attribute.  (Bug#60750)

* lisp/international/mule.el (define-coding-system): Doc fix.

* test/src/coding-tests.el (coding-tests): Use
'with-coding-priority' instead of 'prefer-coding-system', as the
latter has global persistent effect and affects further tests.