mcrouter/lib/fbi/cpp/test/TrieTests.cpp: avoid shadowing warnings
mcrouter/lib/fbi/cpp/test/TrieTests.cpp: avoid shadowing warnings

Summary:
Fix warnings exposed by gcc-4.9.x's -Wshadow-compatible-local
I plan to enable this for all of fbcode, soon.
See t13698406 for justification.

Rename inner "i" to "j".

This avoids the following errors:

  mcrouter/lib/fbi/cpp/test/TrieTests.cpp:238:14: error: declaration of 'i' shadows a previous local [-Werror=shadow-compatible-local]
  mcrouter/lib/fbi/cpp/test/TrieTests.cpp:235:12: error: shadowed declaration is here [-Werror=shadow-compatible-local]

Reviewed By: andreazevedo

Differential Revision: D4656488

fbshipit-source-id: 9fa1cd2f80a72fd1f913204835e8637b37f13428

facebook/mcrouter,facebook/mcrouter,facebook/mcrouter,facebook/mcrouter