pretty-print: add selftest of pp_format's stack

gcc/ChangeLog:
	* pretty-print-format-impl.h (pp_formatted_chunks::get_prev): New
	accessor.
	* pretty-print.cc (selftest::push_pp_format): New.
	(ASSERT_TEXT_TOKEN): New macro.
	(selftest::test_pp_format_stack): New test.
	(selftest::pretty_print_cc_tests): New.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>