Add e2e tests for Plotting Service - Plotting Service

We have no e2e tests.

We should test all endpoints present, but the various find_file endpoints may be worth ignoring as they are migrating to FIA-API.

This may end up just being the Health Check, and that the /data and /meta endpoints function for .nxs files, this last point feels a bit poor form due to testing another library but it would be nice to know.