[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.

GitOrigin-RevId: d0a260b1d124dcad62102011aa80df5f868d28f7
1 file changed