Remove mocking in eum_l2_grib reader tests

[strandgren]

Feature Request

Nowadays the preferred approach is to write a synthetic testing file to tmp_path and work with that instead. However the unit tests for the eum_l2_grib reader (test_eum_l2_grib.py) still uses mocking for creating test files. It shall be assessed if we can write synthetic test files also for the grib format and therefor get rid of the mocking.