TST: add remote-data tests for the esa.euclid module

[bsipocz]

At the time of writing the module (#3216) the Euclid DR1 is not yet public, so accessing the server from CI is not feasible.

Revisit the issue and add remote-tests (in addition to the currently present mocked tests) once the DR is out.

[bsipocz]

cc @cosmoJFH - as Q1 is out now, could we add some remote tests, and remove the doctest skips from the documentation? It would help picking up any inconsistencies, such as #3279