arrow

ARROW-12214: [Rust][DataFusion] Add tests for limit

I was debugging another issue (not a bug in DataFusion I don't think) but noticed there wasn't any coverage for LIMIT in exec.rs, so I figured I would add some.

(well really I was writing a test to trigger what I thought was a bug in DataFusion -- lol)

Closes #9897 from alamb/limit_fix

Authored-by: Andrew Lamb <andrew@nerdnetworks.org>
Signed-off-by: Andrew Lamb <andrew@nerdnetworks.org>