"If {A} attacks, {B} attack if able" effects are impossible to implement correctly

Attack requirement effects are currently only checked once per combat, before the active player has declared any attackers. This is good enough for most requirements, but it doesn't work for effects that depend on which other creatures are attacking.

Affected cards:

[[Ekundu Cyclops]] (not implemented in xmage, see https://github.com/magefree/mage/pull/8594)
[[Magnetic Web]] (not implemented in xmage)
[[Viashino Bey]] (currently implemented, incorrectly, as a triggered ability)
[[War's Toll]] (currently implemented, incorrectly, as a *restriction* effect)