tests/sys/fs/fusefs: include iomanip header

io.cc relies on std::setw(..), which is exported by the iomanip C++
header. Newer versions of GoogleTest don't export this header, so add
the explicit include.

This unbreaks the build with GoogleTest 1.15.2.

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47194