[libc] [UnitTest] Create death tests

Summary: This patch adds `EXPECT_EXITS` and `EXPECT_DEATH` macros for testing exit codes and deadly signals. They are less convoluted than their analogs in GTEST and don't have matchers but just take an int for either the exit code or the signal respectively. Nor do they have any regex match against the stdout/stderr of the child process.

Reviewers: sivachandra, gchatelet

Reviewed By: sivachandra

Subscribers: mgorny, MaskRay, tschuett, libc-commits

Differential Revision: https://reviews.llvm.org/D74665