tests/stdio: normalize whitespace

Move the references to PRs from code comments to the test description. Once
ATF has the ability to output the metadata in the HTML reports, it should be
easy to traverse between releng and gnats -rep

Move the references to PRs from code comments to the test description. Once
ATF has the ability to output the metadata in the HTML reports, it should be
easy to traverse between releng and gnats -reports via links.

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Fix format strings to properly quote %.

Split out 't_printf' and 't_scanf' from 't_format' to gain the common
"functional scope" for the test files.