Use `Nextsim` namespace for XIOS tests

Fixes #589

[x] Defined the tests that specify a complete and functioning change (It may help to create a design specification & test specification)
[x] Implemented the source code change that satisfies the tests
[x] Documented the feature by providing worked example
[x] Updated the README or other documentation
[x] Completed the pre-Request checklist below

Minor change, putting the XIOS tests in the Nextsim namespace to avoid multiple uses of Nextsim::.

[x] The requirements of this pull request are fully captured in an issue or design specification and are linked and summarised in the description of this PR
[x] No new warnings are generated
[x] The documentation has been updated (or an issue has been created to track the corresponding change)
[x] Methods and Tests are commented such that they can be understood without having to obtain additional context
[x] This PR/Issue is labelled as a bug/feature/enhancement/breaking change
[x] File dates have been updated to reflect modification date
[x] This change conforms to the conventions described in the README