[BUG] `forget_vpn()` incorrectly returns success for non-existent connections

[cachebag]

We just need to update the tests to reflect that the forget/delete operation of a saved connection is idempotent, per #200

Note: the tests won't fail in CI since they require a running NetworkManager instance. To reproduce, run tests locally.

failures:

---- test_forget_nonexistent_vpn stdout ----

thread 'test_forget_nonexistent_vpn' (914549) panicked at nmrs/tests/integration_test.rs:956:5:
Forgetting non-existent VPN should return error
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

---- forget_returns_no_saved_connection_error stdout ----

thread 'forget_returns_no_saved_connection_error' (914526) panicked at nmrs/tests/integration_test.rs:793:13:
Expected error, got success


failures:
    forget_returns_no_saved_connection_error
    test_forget_nonexistent_vpn

test result: FAILED. 40 passed; 2 failed; 0 ignored; 0 measured; 0 filtered out; finished in 10.32s

error: test failed, to rerun pass `-p nmrs --test integration_test`