Speed up tests by stubbing out metadata file queries

[stuartlynn]

We should rewrite the metdata and parquets tests to use http_mock to return fake metadata which will remove the need for an active internet connection to run the test and speed up the test suite.

We already have a model of this in the geometry test suite

https://github.com/Urban-Analytics-Technology-Platform/popgetter-cli/blob/6a5224e8c626b0f47afcdf0369578d120bcc17a9/src/geo.rs#L136-L151

Should be easy enough to port over