[region-isolation] Add unittests for SILOptimizer to top level unittest CMakeLists.txt

I forgot to do this when moving the unittests so we were not building/running them.

Author: gottesmm

unittests/CMakeLists.txt

@@ -16,6 +16,7 @@ if(SWIFT_INCLUDE_TOOLS)
   add_subdirectory(Remangler)
   add_subdirectory(Sema)
   add_subdirectory(SIL)
+  add_subdirectory(SILOptimizer)
   add_subdirectory(SwiftDemangle)
 
   add_subdirectory(Threading)