FixedTarget mock is active even when remote-data flag is used

[bmorris3]

In #62, we came up with a mock for FixedTarget.from_name when the internet is not accessible during tests. That mocked from_name method currently gets called during the tests whether or not there is an internet connection, but should only be used when the --remote-data flag is not used.

[eteq]

I'd suggest this approach: Write two tests, one which is marked with @remote_data, and the other with @pytest.mark.skipif('...'). Most of the "guts" of the test can be in a private function called by each of those tests, and you can just use the two versions to do the mocking or not.

The one catch here is the ... in skipif. I'm not exactly sure how to ask pytest "is remote_data on or not". If you look deep into the astropy testing machinery, there's a line that does if not config.getoption('remote_data')... but the part I'm not sure about is how to get a config object actually in a testing module (the usage in the testing machinery is inside a plugin). There must be some way to do so, but I don't have time right now to delve through the pytest docs to find out...