More integration tests

[EddyCMWF]

New tests:

• Test URL dataset with area selection
• Test MARS dataset with netCDF conversion
• Real MARS datasets with and without netcdf conversion:
  • reanalysis-era5-single-levels
  • seasonal-original-single-levels
  • cams-global-atmospheric-composition-forecasts

Also includes a fixture for checking that the dataset exists in the catalogue for testing. This is particularly useful for the CAMS dataset, which will not be visible in a default deployment. Therefore, it is recommended that dev stacks have all datasets visible via the api-url to ensure that all tests are executed.

[FussyDuck]

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