Update METIS background flux tests

[hugobuddel]

There is a disabled test for the METIS background flux

irdb/METIS/tests/norun_test_ScopeSim_MET_IMG_LM.py

Line 94 in 601b236

def test_background_level_is_around_roys_level(self, filter_name, expected_phs):

This test should be updated with the numbers from E-TNT-MPIA-MET-1004 "Detector flux levels in METIS".

E.g. for L band, a temperature of 9 degree C, airmass of 1.3 and pwv of 2.5, we should get about 275k electro-photons, and ScopeSim gives about 373k electro-photons.