tests: isolate FromContextTest from ambient TLS env vars

[ricardobranco777]

DOCKER_TLS_VERIFY/DOCKER_CERT_PATH leaking from the host running the tests (e.g. openQA) broke from_env()'s base_url scheme and caused TLSParameterError. Pop both in setUp alongside DOCKER_HOST, etc.

Otherwise tests fail on openQA:
https://openqa.opensuse.org/tests/6180415

# Test messages # test_from_env_docker_host_overrides_context
# failure: 

AssertionError: assert 'https://192.168.59.103:2375' == 'http://192.168.59.103:2375'
  
  - http://192.168.59.103:2375
  + https://192.168.59.103:2375
  ?     +
tests/unit/client_test.py:302: in test_from_env_docker_host_overrides_context
    assert client.api.base_url == 'http://192.168.59.103:2375'
E   AssertionError: assert 'https://192.168.59.103:2375' == 'http://192.168.59.103:2375'
E     
E     - http://192.168.59.103:2375
E     + https://192.168.59.103:2375
E     ?     +

# Test messages # test_from_env_use_context_false_skips_context
# failure: 

docker.errors.TLSParameterError: If using TLS, the base_url argument must be provided.. TLS configurations should map the Docker CLI client configurations. See https://docs.docker.com/engine/articles/https/ for API details.
tests/unit/client_test.py:308: in test_from_env_use_context_false_skips_context
    client = docker.from_env(
docker/client.py:110: in from_env
    return cls(
docker/client.py:48: in __init__
    self.api = APIClient(*args, **kwargs)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
docker/api/client.py:123: in __init__
    raise TLSParameterError(
E   docker.errors.TLSParameterError: If using TLS, the base_url argument must be provided.. TLS configurations should map the Docker CLI client configurations. See https://docs.docker.com/engine/articles/https/ for API details.