testgen: Helpers to generate conflicting blocks

[hu55a1n1]

Description

Would be nice to have helper functions to generate conflicting light blocks. I am trying to test IBC misbehaviour handling and I need to generate two valid headers (i.e. headers that would pass light client verification) for the same height.

Definition of "done"

Maybe something like ->

impl LightBlock {
    pub fn next_equivocal(&self) -> (Self, Self) { /* ... */ }

    /* ... */
}