[scudo][standalone] Enable death tests on Fuchsia

zxtest doesn't have `EXPECT_DEATH` and the Scudo unit-tests were
defining it as a no-op.

This enables death tests on Fuchsia by using `ASSERT_DEATH` instead.
I used a lambda to wrap the expressions as this appears to not be
working the same way as `EXPECT_DEATH`.

Additionnally, a death test using `alarm` was failing with the change,
as it's currently not implemented in Fuchsia, so move that test within
a `!SCUDO_FUCHSIA` block.

Differential Revision: https://reviews.llvm.org/D94362