private static void assertEntryEquals(byte[] expectedKey, byte[] expectedValue, Entry<byte[], byte[]> entry) {
    assertArrayEquals(expectedKey, entry.getKey());
    assertArrayEquals(expectedValue, entry.getValue());
  }
Concatenates bytes for {@code prefix} with bytes for {@code i}.
If this is called multiple times with the same {@code prefix} and any {@code i}, then this needs to return a byte[]
of the same length for each call.
samza-kv-inmemory/src/test/java/org/apache/samza/storage/kv/inmemory/TestInMemoryKeyValueStore.java