[scudo] Fix memtag tests.

A few memtag tests were using EXPECT_DEBUG_DEATH which executes the
statement unless NDEBUG is set. This meant the statement would
cause an abort, but the EXPECT_DEATH part was not done. Change these
to simply EXPECT_DEATH, these tests already verify that SCUDO_DEBUG
is set before checking for DEATH.

Reviewed By: Chia-hungDuan

Differential Revision: https://reviews.llvm.org/D155538