There are several independent groups of tests in here. It'd help to see the structure if they were in separate classes, with a comment for each. An additional benefit would be you could then shorten some of the method names. For example:
```python
# Tests for LBTIM=001 (Gregorian, valid at T1)
class TestLBTIM_001:
    def _check
    def test_xy
    def test_non_time_cross_section
    def test_time_cross_section

# Tests for LBTIM=011 (Gregorian, forecast from T2 valid at T1)
# etc...
```