[gwp_asan] Exclude recoverable tests on Fuchsia

Signal handlers used by recoverable tests are unsupported on Fuchsia.

Exclude the set of tests that test recoverable code paths (i.e.
BacktraceGuardedPoolAllocator tests in recoverable.cpp) and always set
the `Recoverable` testing bool to `false` on the Fuchsia platform.

Differential Revision: https://reviews.llvm.org/D149311