TST: Revisit xpassed tests and remove xfail from them

[pllim]

Like #6665 but looks like different tests are affected now. Haven't had time to track down exactly which ones. (Would be easier to run tests locally on master with xpassed handled as failures.)

I set the following in my local copy of setup.cfg in the master checkout:

[pytest]
xfail_strict=true

Then, I ran the tests with --remote-data option and found the following marked as xfail to not be failing anymore:

__________________________ test_transforms_diff[sc1] ___________________________
[XPASS(strict)] 
__________________________ test_transforms_diff[sc3] ___________________________
[XPASS(strict)] 

This is the test function in test_sky_coord_velocities.py:

@pytest.mark.xfail
def test_transforms_diff(sc):
    # note that arguably this *should* fail for the no-distance cases: 3D
    # information is necessary to truly solve this, hence the xfail
    trans = sc.transform_to(PrecessedGeocentric(equinox='B1975'))
    assert isinstance(trans.frame, PrecessedGeocentric)

git blame points to @eteq for that implementation.

Also, as @bsipocz found:

# at the time of this writing, sky offset frames don't support velocities
# TODO: make them actually work
@pytest.mark.xfail
def test_skyoffset_frame(sc):

    sc.skyoffset_frame()

p.s. https://travis-ci.org/astropy/astropy/jobs/332937023 reported 6 xpassed but I only found 2 locally in 3.0RC1. Maybe #7065 would make tracking these down much easier.

[pllim]

I looked at test_transforms_diff a bit more and suspect that the xpass was introduced back in #6944 where it was first written (see https://travis-ci.org/astropy/astropy/jobs/320521988 L4487-L4488):

astropy/coordinates/tests/test_sky_coord_velocities.py .........x...x... [ 16%]
X...x.x.x...x...X...x                                                    [ 16%]

This is because two of the 4 test cases do have distance and should not have failed. PR forthcoming.