[GWP-ASan] Port tests to Fuchsia

This modifies the tests so that they can be run on Fuchsia:
- add the necessary includes for `set`/`vector` etc
- do the few modifications required to use zxtest instead og gtest

`backtrace.cpp` requires stacktrace support that Fuchsia doesn't have
yet, and `enable_disable.cpp` currently uses `fork()` which Fuchsia
doesn't support yet. I'll revisit this later.

I chose to use `harness.h` to hold my "platform-specific" include and
namespace, and using this header in tests rather than `gtest.h`,
which I am open to change if someone would rather go another direction.

Differential Revision: https://reviews.llvm.org/D91575