Use mocking for get_survey() tests

[Bisaloo]

Currently, get_survey() tests are skipped on CRAN and on CI, which means they might rarely run and issues could easily go unseen.

socialmixr/tests/testthat/test-surveys.r

Lines 4 to 6 in fa4d164

	skip_if_no_zenodo()
	skip_on_cran()
	skip_on_ci()

Instead, we should add mocking to run these tests to make sure we don't break get_survey(). Mocked resources should still be regularly updated to detect possible changes on zenodo's end.

Useful resource: https://books.ropensci.org/http-testing/

[Bisaloo]

Might be related to #51 depending on the approach we take there.