test: some prop224 unit tests fail only sometimes

Diagnostic from nickm.

11:44 <+nickm> dgoulet, asn: There are two bugs in test_hs_service.c that make it fail sometimes
11:44 <+nickm> on line 541
11:45 <+nickm> bug 1: you're setting one byte of the public key to 0x42.  But that byte might already be 0x42!
11:45 <+nickm> Instead use xor
11:45 <+nickm> bug 2: You are changing the key of an item that's already inserted in the hashtable.
11:45 <+nickm> That can fail when you go to  insert the item with the changed key
11:46 <+nickm> because with probability 1/N_BUCKETS, the item with the changed key will go into the same bucket as it went into the first time...