[flang] Harmonize standalone gtest support with other projects

Use the CMake rules of unittest libraries via `add_subdirectory()`
when using standalone builds, similarly to how e.g. Clang does that.
Besides making things more consistent, I hope this also unblocks D137035
and effectively we'll be able to simplify things further and have LLVM
semi-automatically include gtest support for us in the future.

Differential Revision: https://reviews.llvm.org/D138573