[libFuzzer] Fix failing test: sigint.test

Summary:
Fix sigint.test by making it require msan rather
than enumerating unsupported platforms.

Reviewers: kcc

Reviewed By: kcc

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D61353

llvm-svn: 359638