jstests/sharding/mrShardedOutput.js needs an index

The test does a non-indexed search for each of 200,000 documents across a shared collection in one of its later steps and the test sometimes times out (the overall test takes over 3 hours). The goal is to make sure that all 200,000 documents exist, not to exercise non-indexed search, so these failures are spurious for the purpose of the test. Adding an index on the search field should eliminate the timeouts.

The latest example:
http://buildbot.mongodb.org/builders/Linux%20RHEL%2064-bit/builds/1077/steps/test_11/logs/stdio
http://buildlogs.mongodb.org/Linux%20RHEL%2064-bit/builds/1077/test/sharding/mrShardedOutput.js