slow test in `TestParallaxCorrectionSceneLoad` due to network access

[gerritholl]

Describe the bug

The test TestParallaxCorrectionSceneLoad.test_no_compute in modifier_tests/test_parallax.py takes more than 8 seconds. This is due to network access to get a TLE:

satpy/satpy/utils.py

Lines 425 to 433 in 6d843f6

	name = cth_dataset.attrs["platform_name"]
	tle = tlefile.read(name)
	es = EarthSatellite(tle.line1, tle.line2, name)
	ts = load.timescale()
	gc = es.at(ts.from_datetime(
	cth_dataset.attrs["start_time"].replace(tzinfo=datetime.timezone.utc)))
	(lat, lon) = wgs84.latlon_of(gc)
	height = wgs84.height_of(gc).to("km")
	return (lon.degrees, lat.degrees, height.value)

The test should provide a toy TLE in some way so that no network access is performed.

To Reproduce

pytest --durations=10 test_parallax.py

Expected behavior

Ideally, all tests should be fast (less than 100 ms at most).

Actual results

9.06s call     satpy/tests/modifier_tests/test_parallax.py::TestParallaxCorrectionSceneLoad::test_no_compute

Environment Info:

• Satpy Version: main

Additional context

The automatic downloading is due to github.com/pytroll/pyorbital/pull/210, but after that one is fixed we still need to fix this test to provide the TLEs somehow.

[pnuu]

The easy fix for the test is to write TLE data to a file and point TLES environment variable to it.

[pnuu]

https://pyorbital.readthedocs.io/en/latest/#tle-files