This file was only under gdb/ currently because it predates the
gdb/unittests/ directory.
gdb/ChangeLog:
2017-09-28 Pedro Alves <palves@redhat.com>
* Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
unittests/common-utils-selftests.c.
(SUBDIR_UNITTESTS_OBS): Add common-utils-selftests.o.
(COMMON_OBS): Remove utils-selftests.o.
* utils-selftests.c: Move to ...
* unittests/common-utils-selftests.c: ... here and rename self
test to "string_printf".