[libFuzzer] Remove hardcoded number of new features in merge_two_step.test.

Summary:
The number of features can be different on different platforms.

This should fixed broken builders, e.g.
http://lab.llvm.org:8011/builders/clang-cmake-aarch64-full/builds/7946

Reviewers: Dor1s

Reviewed By: Dor1s

Subscribers: kristof.beyls, delcypher, #sanitizers, llvm-commits

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D67458

llvm-svn: 371647