[flang] update character tests to use gtest

Move character tests to gtest, according to reviews from revision
D97349. Create a new temporary directory parallel to old runtime
unittests directory to facilitate the transition. Once patches for all
tests have been accepted using GTest, the old directory may be removed.
The new directory is required because LLVM's CMake unit test
infrastructure requires that the test depends on all source files in
the `CMAKE_CURRENT_SOURCE_DIR` directory.

Reviewed By: awarzynski

Differential Revision: https://reviews.llvm.org/D97403