ExpectedState addStoreSeg(String storeId, long size, int count) {
      Map<Long, Integer> sizeToNumFiles = storeReserveMap.getOrDefault(storeId, new HashMap<>());
      sizeToNumFiles.put(size, count);
      storeReserveMap.put(storeId, sizeToNumFiles);
      return this;
    }
Update the storeReserveMap with an entry for a certain store and certain file size.
@param storeId the id of store which the segment file is associated with.
@param size the file size
@param count the expected number of files.
@return {@code this}
ambry-store/src/test/java/com/github/ambry/store/DiskSpaceAllocatorTest.java