[libc] Add [EXPECT|ASSERT]_[TRUE|FALSE] unittest macros.

Also, other EXPECT_* and ASSERT_* macros have been extended to accept
bool values.

Reviewers: abrachet, gchatelet

Subscribers: MaskRay, tschuett, libc-commits

Tags: #libc-project

Differential Revision: https://reviews.llvm.org/D73668