attempt to fix auto_roll_logger_test

Summary:
auto_roll_logger_test fails from time to time. I wasn't able to repro
the issue but by looking at the code, it seems like the initial ctime_
value can be set to the boundary of the second so it may still have a
chance to get rolled when interval is set to 1 second.

```
util/auto_roll_logger_test.cc:120: failed: 118 > 708
==19470== Syscall param msync(start) points to unaddressable byte(s)
==19470==    at 0x4E46CE0: __msync_nocancel (in
/usr/local/fbcode/gcc-4.8.1-glibc-2.17/lib/libpthread-2.17.so)
==19470==    by 0x584EFB: access_mem (Ginit.c:137)
==19470==    by 0x5834E3: _ULx86_64_access_reg (libunwind_i.h:162)
==19470==    by 0x585601: apply_reg_state (Gparser.c:742)
==19470==    by 0x5866BE: _ULx86_64_dwarf_find_save_locs (Gparser.c:883)
==19470==    by 0x584550: _ULx86_64_dwarf_step (Gstep.c:34)
==19470==    by 0x583653: _ULx86_64_step (Gstep.c:71)
==19470==    by 0x583FD2: _ULx86_64_tdep_trace (Gtrace.c:217)
==19470==    by 0x5831C3: backtrace (backtrace.c:69)

Test Plan: ./auto_roll_logger_test

Reviewers: sdong, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D21951