Allow tests to provide a Kyuafile when they relied on auto-generation.

When generating a Kyuafile in the KYUAFILE=auto case, use a filename
that is unlikely to clash with the filename used by explicitly-provided
Kyuafiles.

This allows a Makefile to set KYUAFILE=yes and provide a Kyuafile in
the same directory when such Makefile was previously relying on
KYUAFILE=auto.

Fixes issues with new Kyuafiles not being picked up in NO_CLEAN builds
(although manual intervention is required once, unfortunately, as
described in UPDATING).

Reviewed by: sjg
MFC after: 1 week