[flang] Create a separate directory for unittests

Some of the regression tests are C programs that act as test harnesses
for the compiler internals as opposed to being Fortran inputs to test
the compiler in action. The former style of tests are analog to LLVM's
unittests and will not use the lit framework.

Change-Id: I0ff10e23f66ff843e8fff4c35cfb6559b9dab762

Original-commit: flang-compiler/f18@2bfddbe8f8898551a28c3ea07b7ae508018f8634
Reviewed-on: https://github.com/flang-compiler/f18/pull/1027
Tree-same-pre-rewrite: false