[Random] Add s4 field to Xoshiro type (#51332)
This PR adds an optional field to the existing `Xoshiro` struct to be
able to faithfully copy the task-local RNG state.
Fixes #51255
Redo of #51271
Background context: #49110 added an additional state to the task-local
RNG. However, before this PR `copy(default_rng())` did not include this
extra state, causing subtle errors in `Test` where `copy(default_rng())`
is assumed to contain the full task-local RNG state.
