pretty-print: fixes to selftests

Add selftest coverage for %{ and %} in pretty-print.cc

No functional change intended.

gcc/ChangeLog:
	* pretty-print.cc (selftest::test_urls): Make static.
	(selftest::test_urls_from_braces): New.
	(selftest::test_null_urls): Make static.
	(selftest::test_urlification): Likewise.
	(selftest::pretty_print_cc_tests): Call test_urls_from_braces.

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