Make print snapshots more robust

[eitsupi]

I think the polars print is affected by the terminal size (?).
This could affect README.md regenerations and snapshot tests, so it may be worth enforcing that the print results are the same in all environments.

[etiennebacher]

Snapshots should all have the same width since we're using testthat 3rd edition (cf r-lib/testthat#1567). Are there some examples where the snapshots width changes?

[eitsupi]

Thanks for pointing that, it seems good!

I don't know the detailed reason, but the snapshot change occurred in this PR as commented below.
#125 (comment)

[etiennebacher]

Is this still an issue?

[eitsupi]

If it reoccurs, I will reopen.