RFC: Change rules for adding/changing test cases

[DJ Delorie]

> I definitely agree with the last point. What we do in the GNAT project is
> to assign a tracking number to every problem report. The test case is filed
> under this tracking number, and all revision history entries (which are 
> always detailed, and include how and why the problem was fixed) also include
> this same tracking number.

As I mentioned in another mail, not all gcc testcases have
corresponding bug reports to refer to, and it's not always efficient
to rely on a database halfway across the Internet to give you clues
about how to fix bugs a testcase catches.