switch for prefer-gdal config in unit tests and integration tests  #257

CI and test environments should have GDAL available to successfully test those code paths, but we also need to test cases where:

• GDAL is not available
• GDAL is not preferred

It would be nice to run tests like in RasterSourceSpec File RasterSource should / interpret no scheme as file:// with and without config rasterframes.prefer-gdal set to both true and false to exercise all those code paths. #169 was ultimately very hard to debug and test because the failing code path only happened when GDAL was not available.

It would also be good to be able to test around conditions like

doNotPreferGdal()
RasterSource(new URI("vsi://vsicurl/https://host.com/assets/file.tif")) should throw

This issue would be closed by demonstrating a pattern for tests like the above and enabling it in relevant parts of the test suites.