test: Add csv reading test for duckdb_read_csv(na.strings = )

[Tmonster]

prompted by this issue
duckdb/duckdb#8590

[krlmlr]

Thanks. It's not immediately obvious what this test tests. Would it help writing a literal CSV string to a file, or pointing to a file that is already part of the codebase?

[Tmonster]

It just tests to make sure null values are properly read from a csv file using the parameter passed when na_strings is set

Potentially some users write to CSV using R and read using duckdb, so it's nice to make sure NA values are respected when the na_strings parameter is passed.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.89%. Comparing base (10b3416) to head (1bc88c6).

❗ Current head 1bc88c6 differs from pull request most recent head de3dd41. Consider uploading reports for the commit de3dd41 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #10   +/-   ##
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  Coverage   85.89%   85.89%           
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  Files         106      106           
  Lines        3602     3602           
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  Hits         3094     3094           
  Misses        508      508           

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[krlmlr]

Thanks!