[scudo] Fix EXPECT_DEATH tests

Put allocate/deallocate next to memory
access inside EXPECT_DEATH block.
This way we reduce probability that memory is not mapped
by unrelated code.

It's still not absolutely guaranty that mmap does not
happen so we repeat it few times to be sure.

Reviewed By: cryptoad

Differential Revision: https://reviews.llvm.org/D102886