[libFuzzer] Add test for SanitizerCoverage working on Mac even with -Wl,-dead_strip.

Summary:
The corresponding asncov change: https://reviews.llvm.org/D53113.

Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=892167

Reviewers: morehouse, kcc, george.karpenkov

Reviewed By: morehouse, george.karpenkov

Subscribers: delcypher, #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D53114

llvm-svn: 344346