tests: don't alter state in $HOME

tests: don't alter state in $HOME

Diego reported a bug where virsh tries to initialize a readline
history directory during 'make check' run as root, but fails
because /root was read-only.

It turns out that I could reproduce this as non-root, by using:

mv ~/.virsh{,.bak}
chmod a-w ~
make check -C tests TESTS=int-overflow
chmod u+w ~
mv ~/.virsh{.bak,}

* tests/int-overflow: Don't trigger interactive mode.
Reported by Diego Elio Pettenò.