Intermittent failure in BackupableDBTest.ChangeManifestDuringBackupCreation on ppc64le

Example here:

> utilities/backupable/backupable_db_test.cc:1600: Failure
> Value of: db_chroot_env_->FileExists(prev_manifest_path).IsNotFound()
> Actual: false
> Expected: true
> terminate called after throwing an instance of 'testing::internal::GoogleTestFailureException'
> what(): utilities/backupable/backupable_db_test.cc:1600: Failure
> Value of: db_chroot_env_->FileExists(prev_manifest_path).IsNotFound()
> Actual: false
> Expected: true
> Received signal 6 (Aborted)
> #0 /lib/powerpc64le-linux-gnu/libc.so.6(gsignal+0x40) [0x2000005cec90] ?? ??:0
> #1 /lib/powerpc64le-linux-gnu/libc.so.6(abort+0x2b4) [0x2000005d11f4] ?? ??:0
> #2 /usr/lib/powerpc64le-linux-gnu/libstdc++.so.6(_ZN9__gnu_cxx27__verbose_terminate_handlerEv+0x204) [0x20000030c324] ?? ??:0
> #3 /usr/lib/powerpc64le-linux-gnu/libstdc++.so.6(+0xa8b54) [0x200000308b54] ?? ??:0
> #4 /usr/lib/powerpc64le-linux-gnu/libstdc++.so.6(_ZSt9terminatev+0x20) [0x200000308c10] ?? ??:0
> #5 /usr/lib/powerpc64le-linux-gnu/libstdc++.so.6(__cxa_rethrow+0x84) [0x2000003090f4] ?? ??:0
> #6 ./backupable_db_test() [0x105d03b8] bool testing::internal::HandleExceptionsInMethodIfSupported(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*) /home/travis/build/facebook/rocksdb/third-party/gtest-1.8.1/fused-src/gtest/gtest-all.cc:3942
> #7 ./backupable_db_test() [0x105c3d90] testing::UnitTest::Run() /home/travis/build/facebook/rocksdb/third-party/gtest-1.8.1/fused-src/gtest/gtest-all.cc:6242
> #8 ./backupable_db_test() [0x10015f00] RUN_ALL_TESTS() /home/travis/build/facebook/rocksdb/./third-party/gtest-1.8.1/fused-src/gtest/gtest.h:22104
> #9 /lib/powerpc64le-linux-gnu/libc.so.6(+0x2309c) [0x2000005b309c] ?? ??:0
> #10 /lib/powerpc64le-linux-gnu/libc.so.6(__libc_start_main+0xb8) [0x2000005b3298] ?? ??:0
> /bin/bash: line 1: 32438 Aborted (core dumped) ./$t
> Makefile:959: recipe for target 'check_some' failed

Seen here: https://travis-ci.org/github/facebook/rocksdb/jobs/662256267