Interaction between XIOS and doctest?

[jwallwork23]

          As mentioned in the docstring for `assertIsClose`, there is a bizarre bug that crops up if I try to call `REQUIRE` here. I derived a minimum failing example [here](https://github.com/nextsimhub/nextsimdg/blob/bugfix731_xios-tests-mfe2/core/test/XiosReadWrite_test.cpp) but figuring out the exact issue was taking a while and I feel that this isn't important to track down right now, unless the reviewer(s) think so. If we are happy to leave it for now then I'll open an issue as a reminder.

Originally posted by @jwallwork23 in #738 (comment)