add test for default tiff ovrview blocksize (#1611)

[tbonfort]

test never added for #1611, doing this now. fails locally, awaiting ci tests.

[rouault]

@tbonfort Any idea why it fails locally for you ? I can only think to GDAL_TIFF_OVR_BLOCKSIZE being defined in your environment

[tbonfort]

it's not the only test to fail, I have 15% failures on tiff_write, not only related to blocksize. It might be because I have a minimal build, but strange nonetheless.

[tbonfort]

For the record, tests where picking up the system version of gdal/python instead of the one I compiled in a non standard directory. When compiling gdal with --prefix=$foo the tests should be ran with PYTHONPATH=$foo/lib/python3/dist-packages LD_LIBRARY_PATH=$foo/lib . I couldn't find this documented anywhere, should it be?

[rouault]

When compiling gdal with --prefix=$foo the tests should be ran with PYTHONPATH=$foo/lib/python3/dist-packages LD_LIBRARY_PATH=$foo/lib . I couldn't find this documented anywhere, should it be?

doc/source/contributing/developer.rst suggests to use . scripts/setdevenv.sh (it might pick the wrong python version if you have both python and python3 in your path though), which avoids to run "make install" when developing.