Use Salesforce instance fixture in tests/unit/test_salesforce.py

[mgardzinski]

Repeating part that needs to be replaced in tests:

  mock_sf_instance = mocker.MagicMock(spec=Salesforce)
  mocker.patch("viadot.sources.salesforce.Salesforce", return_value=mock_sf_instance)

Suggested change for fixture:

@pytest.fixture
def mock_salesforce(mocker):
    """Fixture to mock the Salesforce instance."""
    mock_sf_instance = mocker.MagicMock(spec=SimpleSalesforce)
    mocker.patch("viadot.sources.salesforce.Salesforce", return_value=mock_sf_instance)
    return mock_sf_instance