[libfuzzer] Temporarily disable unittests failing on Windows.

Summary:
Patch by Jonathan Metzman (@metzman).


Reviewers: morehouse, Dor1s

Reviewed By: Dor1s

Subscribers: kcc, delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D51712

llvm-svn: 341514