test: create centralized test fixture library (#311)

[mikwansa]

Closes #311

Summary

Creates a shared test fixture library at tests/common/mod.rs to eliminate duplicated setup code across 20+ integration test files.

Helpers provided

• setup_env() — Env with default ledger and mock auths
• setup_contract() — full env + deployed NebulaNomadContract + client
• advance_ledger(env, count) — advance by N ledgers
• advance_time(env, seconds) — advance timestamp only
• set_ledger(env, seq, ts) — exact ledger state
• generate_addresses(env, n) — bulk Address generation
• test_hash(env, seed) — 32-byte hash

Usage

In any test file:

mod common;
use common::*;

fn my_test() {
    let (env, client, admin) = setup_contract();
    // ...
}

Next steps

Migrate existing test files to use the shared library (separate PRs to keep changes focused).

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