Made CSV and JSON tests not use files.

[jorgecarleitao]

This makes testing easier and allows miri to check them.

Also greatly simplified them.

[codecov]

Codecov Report

Merging #290 (e4b77b1) into main (a5f0575) will decrease coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #290      +/-   ##
==========================================
- Coverage   77.36%   77.25%   -0.12%     
==========================================
  Files         315      315              
  Lines       20936    20791     -145     
==========================================
- Hits        16198    16062     -136     
+ Misses       4738     4729       -9     

Impacted Files	Coverage Δ
src/io/json/read/infer_schema.rs	88.66% <ø> (ø)
src/io/json/read/reader.rs	75.67% <ø> (-14.87%)	⬇️
tests/it/io/csv/read.rs	100.00% <100.00%> (ø)
tests/it/io/json/mod.rs	100.00% <100.00%> (ø)
tests/it/io/json/read.rs	100.00% <100.00%> (ø)
src/io/json/read/deserialize.rs	70.86% <0.00%> (-2.37%)	⬇️
src/array/mod.rs	47.73% <0.00%> (+1.50%)	⬆️
src/array/dictionary/mod.rs	72.50% <0.00%> (+7.50%)	⬆️
src/array/boolean/mutable.rs	58.51% <0.00%> (+12.59%)	⬆️

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