Run fsync tests on mutable filesystem.

Run fsync tests on mutable filesystem.

The purpose of those tests is to ensure fsync succeeds for
directories. However, they try to call fsync for / (the root
filesystem) that is usually mounted read-only, which does not
make sense because the whole point of fsync is to flush
uncommitted changes to the filesystem.

In fact, some well-known read-only filesystems (e.g. squashfs)
does not support fsync, so these tests fail if we use such
filesystems as the root filesystem.

This patch changes the tests to call fsync against /data
instead. /data is a user data partition and should be always
mounted read-write.

Bug: 28681455
Change-Id: I9314dad6f8d54c66401714af0f66e782ebd8e319