Merge the string_printf and string_vprintf tests, running them all
against both functions.
gdb/ChangeLog:
2017-10-30 Pedro Alves <palves@redhat.com>
* unittests/common-utils-selftests.c (format_func): New typedef.
(string_printf_tests, string_vprintf_tests): Tests factored out
and merged to ...
(test_format_func): ... this new function.
(string_printf_tests, string_vprintf_tests): Reimplement on top of
test_format_func.