Add BigInt inline tests for unsafe math and overflow (#333)

## Type of change

<!--Delete points that do not apply-->

- Improvement (refactoring, restructuring repository, cleaning tech
debt, ...)

## Changes

The following changes have been made:

- Adds tests for expected behavior from `Add` when unsafe math and
overflow are enabled
- Adds tests for expected behavior from `Multiply` when unsafe math and
overflow are enabled
- Adds tests for expected behavior from `Subtract` when unsafe math and
overflow are enabled

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [x] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.
- [x] I have updated the changelog to reflect the changes on this PR.

Author: bitzoic

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 - [#312](https://github.com/FuelLabs/sway-libs/pull/312) Implements `TotalOrd` trait for `I8`, `I16`, `I32`, `I64`, `I128`, and `I256`.
 - [#326](https://github.com/FuelLabs/sway-libs/pull/326) Introduces the Big Integers Library with the `BigUint` type.
 - [#329](https://github.com/FuelLabs/sway-libs/pull/329) Introduce the Sparse Merkle Proof Library.
+- [#333](https://github.com/FuelLabs/sway-libs/pull/333) Adds `BigInt` inline tests for expected behavior on overflow and unsafe math.
 
 ### Changed