[libFuzzer] Disable interceptor tests on non-Linux platforms.

Summary: libFuzzer's interceptor support added in 831ae45e currently only works on Linux. This patch disables the test cases added as part of that commit on non-Linux platforms.

Reviewers: morehouse, hctim

Subscribers: #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D84434