[cmake] Make gtest macro definitions a part the library interface

These definitions are needed by any file which uses gtest. Previously we
were adding them in the add_unittest function, but over time we've
accumulated libraries (which don't go through add_unittest) building on
gtest and this has resulted in proliferation of the definitions.

Making this a part of the library interface enables them to be managed
centrally. This follows a patch for -Wno-suggest-override (D84554) which
took a similar approach.

Differential Revision: https://reviews.llvm.org/D84748