test: Generate distinct hashes

Gen::just is the constant generator. Don't just return that!

(cherry picked from commit 8406da28773f050e00a006e4812e3ecbf919a2a9)
Change-Id: Ibfd0bd40f90942077a4720086ce0cd3bfabef79d

tests/unit/libutil-support/tests/hash.cc

@@ -11,10 +11,17 @@ using namespace nix;
 
 Gen<Hash> Arbitrary<Hash>::arbitrary()
 {
+    Hash prototype(htSHA1);
+    return
+        gen::apply(
+            [](const std::vector<uint8_t> & v) {
                 Hash hash(htSHA1);
-    for (size_t i = 0; i < hash.hashSize; ++i)
-        hash.hash[i] = *gen::arbitrary<uint8_t>();
-    return gen::just(hash);
+                assert(v.size() == hash.hashSize);
+                std::copy(v.begin(), v.end(), hash.hash);
+                return hash;
+            },
+            gen::container<std::vector<uint8_t>>(prototype.hashSize, gen::arbitrary<uint8_t>())
+        );
 }
 
 }