Improve unit tests

[simonkassel]

We would like to improve unit tests in some portions of the code base. Here are some locations that could use additional testing. Feel free to not limit yourself to these examples.

• Test validation on invalid stac examples https://github.com/azavea/pystac/blob/develop/tests/validation/test_validate.py
• can_extend method of base extension object https://github.com/azavea/pystac/blob/2605f271da7e9e267cf706a2c72e29d1f32785ce/pystac/extensions/base.py#L208
• TemporalExtent constructor edge cases, including invalid interval arguments https://github.com/azavea/pystac/blob/2605f271da7e9e267cf706a2c72e29d1f32785ce/pystac/collection.py#L359
• Making an href relative when start_is_dir=True https://github.com/azavea/pystac/blob/2605f271da7e9e267cf706a2c72e29d1f32785ce/pystac/utils.py#L42
• geometry_to_bbox https://github.com/azavea/pystac/blob/2605f271da7e9e267cf706a2c72e29d1f32785ce/pystac/utils.py#L161
• Methods within ResolvedObjectCollectionCache https://github.com/azavea/pystac/blob/2605f271da7e9e267cf706a2c72e29d1f32785ce/pystac/cache.py#L193
• Tests in EO extension apply method https://github.com/azavea/pystac/blob/develop/pystac/extensions/eo.py
• For several extensions, construct extension object from an item that does not already have extension listed in its stac_extensions property (https://github.com/azavea/pystac/blob/2605f271da7e9e267cf706a2c72e29d1f32785ce/pystac/extensions/view.py#L23)
• Edge cases for StacVersionRange comparisons https://github.com/azavea/pystac/blob/5a2f9278e76cbfc03643ebd0ee90bc98ab03aaeb/tests/serialization/test_identify.py#L55
• Setting non-null collections on SingleFileSTACCAtalogExt https://github.com/azavea/pystac/blob/5a2f9278e76cbfc03643ebd0ee90bc98ab03aaeb/pystac/extensions/single_file_stac.py#L125

[whatnick]

This issue should be updated to reference new home, possibly broken up into sub-issues for each section that requires unit tests.

[lossyrob]

Closing as this list is out of date.