Add tests for device CLI skeleton

tests/cli/device/test_nucypher_device.py

@@ -0,0 +1,25 @@
+import pytest
+from nucypher.cli.main import nucypher_cli
+
+
+def test_device_cli_backend_errors(click_runner):
+    with pytest.raises(RuntimeError):
+        device_args = ('device', 'init')
+        click_runner.invoke(nucypher_cli, device_args,
+                            catch_exceptions=False)
+
+    # TODO: When multiple device support is added, add test to show you can't
+    # call `nucypher device` with multiple device flags.
+
+    device_args = ('device', 'bad arg')
+    res = click_runner.invoke(nucypher_cli, device_args,
+                              catch_exceptions=False)
+    assert res.exit_code == 2
+
+
+def test_device_cli_init_trezor(click_runner, mock_trezorlib):
+    trezor_init_args = ('device', 'init', '--trezor')
+
+    with pytest.raises(NotImplementedError):
+        click_runner.invoke(nucypher_cli, trezor_init_args,
+                            catch_exceptions=False)