atf_python: Run vnet handlers in $HOME

When kyua runs a test, it creates a temp directory and sets $HOME to
point to it. Tests are run with the cwd set to that temp directory.

When a process attaches to a jail, its cwd is set to the root of the
jail. Modify atf_python to cd to $HOME instead, so that it's easier for
tests to share files.

Reviewed by: zlei, ngie
MFC after: 2 weeks
Sponsored by: Stormshield
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D54971