include from_vec only when running tests

rust/solve/src/dpll.rs

@@ -8,6 +8,7 @@ fn empty_vars() -> BoundVars {
    BTreeSet::new()
}
#[cfg(test)]
fn from_vec(v: Vec<Var>) -> BoundVars {
    let mut vars = empty_vars();
    vars.extend(v);