[Ballista] Fix regression in roundtrip_logical_plan_custom_ctx test

[andygrove]

Describe the bug
PR apache/datafusion#2537 fixed an issue where the csv scan methods were using the full URI instead of the path when serializing csv scans, which was not consistent with the way other scans worked (parquet, avro, json). Making this consistent led to a regression in roundtrip_logical_plan_custom_ctx so the test was ignored for now.

We should re-enable this test.

To Reproduce
Run roundtrip_logical_plan_custom_ctx.

Expected behavior
Functionality should be consistent between file types.

Additional context
None

[andygrove]

@thinkharderdev @mingmwang I could use some help with this if you have time to look.

[thinkharderdev]

Yeah, the issue here is that the serde logic uses the file scheme to resolve the ObjectStore. So we were relying on the test scheme to resolve the TestObjectStore. Why would we not serialize the entire file URI?