d: Add UTF BOM tests to gdc.dg testsuite

d: Add UTF BOM tests to gdc.dg testsuite

Some of these are part of the upstream DMD `gdc.test' testsuite, but
they had been omitted because they get mangled by the lib/gdc-utils.exp
helpers when parsing and staging the tests. Translate them over to the
gdc.dg testsuite instead.

gcc/testsuite/ChangeLog:

	* gdc.dg/bom_UTF16BE.d: New test.
	* gdc.dg/bom_UTF16LE.d: New test.
	* gdc.dg/bom_UTF32BE.d: New test.
	* gdc.dg/bom_UTF32LE.d: New test.
	* gdc.dg/bom_UTF8.d: New test.
	* gdc.dg/bom_characters.d: New test.
	* gdc.dg/bom_error_UTF8.d: New test.
	* gdc.dg/bom_infer_UTF16BE.d: New test.
	* gdc.dg/bom_infer_UTF16LE.d: New test.
	* gdc.dg/bom_infer_UTF32BE.d: New test.
	* gdc.dg/bom_infer_UTF32LE.d: New test.
	* gdc.dg/bom_infer_UTF8.d: New test.