tests: Make /etc/alternatives work in VM

tests: Make /etc/alternatives work in VM

In recent Debian versions, ebtables is an alias managed by
the alternatives(8) mechanism. This means /usr/sbin/ebtables
is a symlink to /etc/alternatives/ebtables, which in turn
links to the real binary.

As we mount a tmpfs over /etc, we cannot access this.

Fix this by bind-mounting the real /etc to /tmp/etc and
adding a symlink from /etc/alternatives to this.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>