Skip log location tests in AppVeyor CI
These new logging tests fail on AppVeyor:
test_server.py::test_stream_logging_source_location[Asyncio]
test_server.py::test_decorator_logging_source_location[Synchronous]
test_server.py::test_decorator_logging_source_location[Asyncio]
test_server.py::test_stream_logging_source_location[Synchronous]
This is due to some difference in environment and the behaviour of the pytest capfd fixture. We have the gitlab CI tests to cover this, so it isn't critical to test under Windows.
The tests are skipped if the environment variable isn't set.
Related issue: #515 (closed)