Convert CLI tests to integration tests, increase overall test coverage, slight change to API url building logistics

[jreiberkyle]

Changes:

• CLI tests are now integration tests, where none of the planet library is mocked, just server responses are mocked. While many of the integration tests are 1:1 with the unit tests, some new tests were added and some existing tests were broken into multiple integration tests. Overall coverage increased.
• Minimum test coverage to pass tests is increased from 95% to 97%
• Change where slashes are in API url building to match API documentation (example pulled from docs below, note how relative paths start with a slash)
  
• URLs are now built using f-strings instead of string concatenation (closes move url creation in the auth and orders clients to f-strings #382)

Closes #327

[sgillies]

@jreiberkyle what would you think about using mock.patch instead of pytest's monkeypatch?

Suggested change

	@pytest.mark.parametrize(
	"pretty,expected",
	[(False, '{"key": "val"}'), (True, '{\n "key": "val"\n}')])
	def test_cli_echo_json(pretty, expected, monkeypatch):
	mock_echo = Mock()
	monkeypatch.setattr(io.click, 'echo', mock_echo)
	@pytest.mark.parametrize(
	"pretty,expected",
	[(False, '{"key": "val"}'), (True, '{\n "key": "val"\n}')])
	@mock.patch('io.click.echo')
	def test_cli_echo_json(mock_echo, pretty, expected, monkeypatch):

[jreiberkyle]

This works beautifully with one change: patch uses the full path name to the module. (see paragraph discussing the target arg in the patch docs)

[sgillies]

I left one suggestion inline for using mock.patch instead of pytest.monkeypatch in the case where we're using mock.Mock.