Add valgrind to pre-commit sandcastle testing

Summary:
Initially I removed "valgrind" from the list since it take too much
time (3+hr) compared to tsan (40 min) when the tests are run in parallel. It is
not effective to run the tests in parallel in sandcastle and tsan takes about
3hrs as well.

Adding valgrind to the list.

Test Plan: Submit this diff and watch the run

Reviewers: sdong, rven

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D53337

arcanist_util/config/FacebookArcanistConfiguration.php

@@ -123,7 +123,7 @@ class FacebookArcanistConfiguration extends ArcanistConfiguration {
 
     // list of tests we want to run in sandcastle
     $tests = array(
-      "unit", "unit_481", "clang_unit", "tsan", "asan", "lite"
+      "unit", "unit_481", "clang_unit", "tsan", "asan", "lite", "valgrind"
     );
 
     // construct a job definition for each test and add it to the master plan