Test: CTS: use secure temporary files for bandwidth and remote tests

Previously, the bandwidth test used the deprecated, insecure mktemp() function.
Now, it uses mkstemp().

Previously, the remote tests called lxc_autogen.sh to create a local copy of
the remote authentication key if needed, but that fails if the user running CTS
does not have write privileges on /etc. Now, it creates a secure temporary file
instead.