ci: run diag manager tests on CI

[romanc]

Description

This is a follow-up from #427 attempting to run pyFMS tests on CI. Doing so currently fails with a segfault when attempting to open diag_manager_single_tile.nc in line

ds = xr.open_mfdataset("diag_manager_single_tile.nc", decode_times=True)

How has this been tested?

All good as long as CI is green ...

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation (e.g. add new modules to docs/docstrings/)
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included

[romanc]

superseded by #459