Move test_target_ops to a separate file

Move test_target_ops to a separate file

There's no need to have all target.h users seeing this type.

Also helps with a follow up patch.

gdb/ChangeLog:
2018-11-30  Pedro Alves  <palves@redhat.com>

	* Makefile.in (COMMON_SFILES): Add test-target.c.
	* gdbarch-selftests.c: Include "test-target.h".
	* regcache.c: Include "test-target.h".
	* target.c (test_target_info, test_target_ops::info): Move to ...
	* test-target.c: ... this new file.
	* target.h (test_target_ops): Move to ...
	* test-target.h: ... this new file.