Improve SimpleWriteTimeoutTest to avoid false alarm.

Summary:
SimpleWriteTimeoutTest has two parts: 1) insert two large key/values
to make memtable full and expect both of them are successful; 2) insert
another key / value and expect it to be timed-out.  Previously we also
set a timeout in the first step, but this might sometimes cause
false alarm.

This diff makes the first two writes run without timeout setting.

Test Plan:
export ROCKSDB_TESTS=Time
make db_test

Reviewers: sdong, ljin

Reviewed By: ljin

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D19461