Add selftest for pretty-print.c

Add selftest for pretty-print.c

gcc/ChangeLog:
	* pretty-print.c: Include "selftest.h".
	(pp_format): Fix comment.
	(identifier_to_locale): Likewise.
	(selftest::test_basic_printing): New function.
	(selftest::assert_pp_format): New function.
	(selftest::test_pp_format): New function.
	(selftest::pretty_print_c_tests): New function.
	* selftest-run-tests.c (selftest::run_tests): Call
	selftest::pretty_print_c_tests.
	* selftest.h (pretty_print_c_tests): New declaration.

From-SVN: r237221