feat(gui, tauri): Accept --testnet flag, default to mainnet #357

Job	Run time
check_stable	47s
bdk_test	1m 8s
docker_tests (happy_path)	1s
build (x86_64-unknown-linux-gnu, ubuntu-latest)	1m 4s
rpc_tests	52s
docker_tests (happy_path_restart_bob_after_xmr_locked)	1s
build (armv7-unknown-linux-gnueabihf, ubuntu-latest)	1m 19s
sqlx_test	44s
docker_tests (happy_path_restart_bob_before_xmr_locked)	1s
build (x86_64-apple-darwin, macos-12)	1m 17s
build (aarch64-apple-darwin, macos-latest)	1m 1s
docker_tests (happy_path_restart_alice_after_xmr_locked)	1s
static_analysis	1m 7s
build (x86_64-pc-windows-msvc, windows-latest)	1m 19s
test (ubuntu-latest)	1s
docker_tests (alice_and_bob_refund_using_cancel_and_refund_command)	40s
test (macos-latest)	57s
docker_tests (alice_and_bob_refund_using_cancel_then_refund_command)	55s
docker_tests (alice_and_bob_refund_using_cancel_and_refund_command_timelock_not_expired)	53s
docker_tests (alice_manually_punishes_after_bob_dead_and_bob_cancels)	1s
docker_tests (punish)	1s
docker_tests (alice_punishes_after_restart_bob_dead)	1s
docker_tests (alice_manually_punishes_after_bob_dead)	55s
docker_tests (alice_refunds_after_restart_bob_refunded)	1s
docker_tests (ensure_same_swap_id)	1s
docker_tests (concurrent_bobs_before_xmr_lock_proof_sent)	1s
docker_tests (concurrent_bobs_after_xmr_lock_proof_sent)	1s
docker_tests (alice_manually_redeems_after_enc_sig_learned)	1s
docker_tests (happy_path_bob_offline_while_alice_redeems_btc)	1s
	15m 12s

Provide feedback