Update cover tests

[cdce8p]

Description:

First preparations for home-assistant/architecture#58
I've removed all test references to the cover services helpers. In addition the following test files are now async:

• cover/test_command_line.py
• cover/test_demo.py
• cover/test_group.py

Since I didn't jet remove the service helpers, this PR does not contain any breaking changes.

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
  If the code does not interact with devices:

[balloob]

If we can, we should just move the Cover service call helpers to tests/components/cover/__init__.py . That way we don't have to change the tests and can implement this faster.

[cdce8p]

@balloob I could remove the Cover service call helpers if you want to. This PR removes all references to them. I just didn't do it since it might break some custom components.

[balloob]

Let's remove them.