Upgrade tests to use mocker patch and mock EchoData object

[leewujung]

Some of the current tests (such those in #1319) run through open_raw and compute_Sv in order to test downstream functionalities that require both the Sv dataset and the correspondning EchoData object. We should upgrade these tests to use mocker patch and mock EchoData objects (I believe the core mechanism is already in place in tests/echodata/utils.py::get_mock_echodata - should probably make it a fixture that can be used elsewhere), so that we are not tied with the particularities of the test .raw files and are just testing the exact operations of these functions.