chore(fixtures): run fixtures on `macos` & `ubuntu`

Description

Added matrix to CI in order to check the fixtures works on both OS

Motivation and Context

macos is pretty common OS and can be compatible with testing framework

In continuation of https://github.com/orhun/rustypaste/pull/200

How Has This Been Tested?

Changelog Entry

### Added

- Check fixtures against MacOS in CI

Types of Changes

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[x] Documentation (no code change)
[ ] Refactor (refactoring production code)
[X] Other Testing

Checklist:

[] My code follows the code style of this project.
[ ] I have updated the documentation accordingly.
[ ] I have formatted the code with rustfmt.
[ ] I checked the lints with clippy.
[ ] I have added tests to cover my changes.
[ ] All new and existing tests passed.

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (adbd67a) 70.23% compared to head (a698d03) 70.23%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #201 +/- ## ======================================= Coverage 70.23% 70.23% ======================================= Files 11 11 Lines 598 598 ======================================= Hits 420 420 Misses 178 178 ``` | [Flag](https://app.codecov.io/gh/orhun/rustypaste/pull/201/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z) | Coverage Δ | | |---|---|---| | [unit-tests](https://app.codecov.io/gh/orhun/rustypaste/pull/201/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z) | `70.23% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Orhun+Parmaks%C4%B1z#carryforward-flags-in-the-pull-request-comment) to find out more.

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orhun / rustypaste