[scudo] Use TestAllocator wrapper to ensure proper cleanup (#160800)
Instead of directly instantiating scudo::Allocator, using the test
TestAllocator wrapper class ensures that unmapTestOnly is called at the
end of the test.
This fixes the issue of QuarantineIterateOverChunks failing on Fuchsia
because of a clobbered TLS pointer left by QuarantineEnabled.