logging: move the fatal_helper test program to test/helpers

Summary:
Move `FatalHelper.cpp` to the new folly/logging/test/helpers directory created
in D13984692.

Reviewed By: yfeldblum, dmaone

Differential Revision: D13994517

fbshipit-source-id: e2b7e4b4f649fed5b882df47424d24e4fc4f717a

folly/logging/test/fatal_test.py

@@ -29,7 +29,7 @@ class FatalTests(unittest.TestCase):
         else:
             build_dir = os.path.join(os.getcwd(), "buck-out", "gen")
             self.helper = os.path.join(
-                build_dir, "folly", "logging", "test", "fatal_helper"
+                build_dir, "folly", "logging", "test", "helpers", "fatal_helper"
             )
 
     def run_helper(self, *args, **kwargs):