Move selftest::test_diagnostic_context to its own header

Move selftest::test_diagnostic_context to its own header

It's useful to not rely on global_dc in selftests, so this patch
moves class selftest::test_diagnostic_context from
diagnostic-show-locus.c to a new header and source file.

gcc/ChangeLog:
	* Makefile.in (OBJS-libcommon): Add selftest-diagnostic.o.
	* diagnostic-show-locus.c: Include "selftest-diagnostic.h".
	(class selftest::test_diagnostic_context): Move to...
	* selftest-diagnostic.c: New file.
	* selftest-diagnostic.h: New file.

From-SVN: r254354