Function prop_join_correctness 

pub fn prop_join_correctness(
    ctx: &TestContext,
    left_keys: Vec<u32>,
    left_vals: Vec<u32>,
    right_keys: Vec<u32>,
    right_vals: Vec<u32>,
) -> Result<(), String>

Property: Hash join result equals the set of all matching pairs.

For each (key, lval) in left and (key, rval) in right where keys match, the result should contain exactly one (key, lval, rval) tuple.