mock_da feature is required to run tests for sov-rollup-starter

[soareschen]

The feature mock_da is enabled by default in the sov-rollup-starter template. When trying to change the default feature to celestia_da, it would result in various compile errors in the test crate.

Some adjustments should be made so that disabling mock_da won't result in compile failure of the tests. For example, the tests can require the sov-mock-da crate non-optionally, or mock tests are made to only compile behind cfg flag if the mock_da feature is enabled.