Do not overwrite cache dumps

Do not overwrite cache dumps

Unless configured otherwise in named.conf, "rndc dumpdb" causes a cache
dump to be written to a file called "named_dump.db" in the working
directory of the given named instance.  Repeatedly using this command
throughout different checks in the cacheclean system test causes cache
dumps for older checks to be overwritten, which hinders failure
diagnosis.  Prevent this by moving each cache dump to a check-specific
location after running "rndc dumpdb".

Furthermore, during the "check flushtree clears adb correctly" check,
dump_cache() is called twice without renaming the resulting files.
Prevent the first cache dump from being overwritten by moving it to a
different file before calling "rndc dumpdb" for the second time.