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MINOR: [Go] Add tests for decimal arithmetic operations (#34849)



### Rationale for this change
There are no tests for arithmetic operations of decimal128 and decimal256 in the project.

Tests cover the following operators:
- Add
- Sub
- Mul
- Div
### What changes are included in this PR?
Add tests for these cases.

### Are these changes tested?
Yes

### Are there any user-facing changes?
No

Authored-by: izveigor <izveigor@mail.ru>
Signed-off-by: Matt Topol <zotthewizard@gmail.com>