TestHashCacheAddContainsHashes flakes fairly regularly when rebasing
PR btcsuite#1684 with:
    txid <txid> wasn't inserted into cache but was found.

With probabilty 1/10^2 there will be no inputs on the transaction. This
reduces the entropy in the txid, and I belive is the primary cause of
the flake.

This resolves the more fundamental flake in the unit tests noted in the
prior commit.

Because multiple unit tests call rand.Seed in parallel, it's possible
they can be executed with the same unix timestamp (in seconds). If the
second call happens between generating the hash cache and checking that
the cache doesn't contain a random txn, the random transaction is in
fact a duplicate of one generated earlier since the RNG state was reset.

To remedy, we initialize rand.Seed once in the init function.