Fix buffer over-reads in the rendcache tests

Fix buffer over-reads in the rendcache tests

The hidden service descriptor cache (rendcache) tests use digest maps
which expect keys to have a length of DIGEST_LEN.

Because the tests use key strings with a length lower than DIGEST_LEN,
the internal copy operation reads outside the key strings which leads to
buffer over-reads.

The issue is resolved by using character arrays with a size of
DIGEST_LEN.

Patch on ade5005853c17b3ae5923c194680442e0f86db4d.