@Test
  public void findKeyTest() throws StoreException {
    for (MockId id : state.allKeys.keySet()) {
      IndexValue value = state.index.findKey(id);
      verifyValue(id, value);
    }
    // search for a non existent key
    MockId nonExistentId = state.getUniqueId();
    verifyValue(nonExistentId, state.index.findKey(nonExistentId));
  }
Tests for {@link PersistentIndex#findKey(StoreKey)}.
Cases:
1. Live keys
2. Expired keys
3. Deleted keys
4. Non existent keys
@throws StoreException
ambry-store/src/test/java/com/github/ambry/store/IndexTest.java