bug_test_rst--benja: Marking failing tests to use ReST

Tuomas' "Handling Bugs" PEG specifies that an RFC 2822-like syntax should be used to mark failing PEGs. However,

• RFC 2822 headers are supposed to be at the beginning of something; putting them at the end of something (as "Handling Bugs" does) isn't really well-specified and needs our own parser.
• These headers aren't meant to be used in docstrings.

Therefore, I propose to use reStructuredText field lists instead. They're designed to be very much like RFC 2822 headers, but can appear anywhere in a reST fragment-- and reStructuredText was designed for use in Python docstrings.

- Benja