Fix decimal in get_testing_kernel_expression and visitor printing

[OussamaSaoudi-db]

What changes are proposed in this pull request?

This PR changes the Decimal in get_testing_kernel_expression to have a precision larger than the scale, which is a requirement of Decimals. Moreover and the existing behaviour may cause errors in engines that check that Decimals are well-formed.

The child list parameter name in visit_literal_struct is renamed to be consistent with the other visitor.

Finally, the printing is patched up in expression_print.h to be consistent with the order in the Decimal struct.

How was this change tested?.

I ran the visit-expression test suite with the new expected output.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.41%. Comparing base (4466509) to head (9fcc430).

Files with missing lines	Patch %	Lines
ffi/src/test_ffi.rs	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #456   +/-   ##
=======================================
  Coverage   78.41%   78.41%           
=======================================
  Files          55       55           
  Lines       11806    11806           
  Branches    11806    11806           
=======================================
  Hits         9258     9258           
  Misses       2041     2041           
  Partials      507      507           

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