Heinz,
Thanks for your comments.
Most of the times, the place to use is the bugs system.
When you submit a bug, add a test case to the bugs report.
If you are providing a test for another bug or a feature, add a comment to the relative bug report and a file containing the test. The same apply to patches.
Generally speaking, a new test case is a patch. So it can be submitted as any other patch, after discussing it in the internals@ mailing list. If the test case refers to a bug, you can add it to the bug report. If it is not a bug, then you can open a feature request (e.g.: "test option X of feature Y") and attach the test case there.
But first discuss it in the mailing list.
Cheers
Giuseppe
--
Giuseppe Maxia, MySQL Community Team Leader at Oracle
http://dev.mysql.com