Minor: update tests in limit_pushdown.rs to insta (#20066)

## Which issue does this PR close?

- Follow on to #20048



## Rationale for this change

While reviewing #20048 and
verifying test coverage, it was hard for me to see the test differences
(b/c the formatting was not great)

## What changes are included in this PR?

Port the tests to use insta rather than `assert_eq`
## Are these changes tested?
Yes, only tests

## Are there any user-facing changes?

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Author: alamb