gcovr: Move fuzz/old.gc* file one level lower

gcovr fails to process fuzz/old.gcda and fuzz/old.gcno files after !7045 (merged) MR with:

(WARNING) GCOV produced the following errors processing /builds/isc-projects/bind9/fuzz/old.gcda:
	Cannot open source file ../../fuzz/old.c
Cannot open source file ../../lib/dns/include/dns/compress.h
Cannot open source file ../../lib/isc/include/isc/buffer.h
...
(gcovr could not infer a working directory that resolved it.)

The fix is to move fuzz/old.gc* files one directory level lower and then fix included header and source file errors.

Validation job: https://gitlab.isc.org/isc-projects/bind9/-/jobs/2919860