[libfuzzer] Fix fuzzer-oom.test on windows and reenable it.

Summary: Patch by Jonathan Metzman (@metzman).

Reviewers: Dor1s, morehouse

Reviewed By: Dor1s

Subscribers: delcypher, llvm-commits, #sanitizers

Differential Revision: https://reviews.llvm.org/D51735

llvm-svn: 341562