Name all DEATH tests using 'DeathTest' suffix.

gtest highly recommends this prefix, and runs death tests first
(https://github.com/google/googletest/blob/master/docs/advanced.md#death-test-naming).
This may help with some spurious bot failures like
https://lab.llvm.org/buildbot/#/builders/169/builds/1290/steps/25/logs/stdio.

Reviewed By: cryptoad, vitalybuka

Differential Revision: https://reviews.llvm.org/D105371