The descr meta-data field is not required anymore

[rillig]

atf-test-case.4 says that descr is required for a test case, while in fact, it was made optional in version 0.8.

While here, atf-test-case.4 promises that descr "will be shown to the user in reports", but when I ran atf-run on NetBSD, none of the test descriptions were listed in the raw report file, which started with application/X-atf-tps; version="3".