test: Upgrade test deps

[benjiro]

This PR

Upgrade all test deps

• Autofixture 4.17 -> 4.18.1
• coverlet 3.1.2 -> 6.0.0
• FluentAssertions 6.7.0 -> 6.12.0
• MSN.Test.Sdk 17.2.0 -> 17.9.0
• NSubstitue 5 -> 5.1
• FlagD Provider 0.1.5 -> 0.1.8
• Xunit 2.4.1 -> 2.6.6

Related Issues

Fixes #197

[codecov[bot]]

Codecov Report

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

Comparison is base (2c237df) 93.50% compared to head (1ebabae) 93.60%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #221 +/- ## ========================================== + Coverage 93.50% 93.60% +0.10% ========================================== Files 23 23 Lines 954 954 Branches 105 104 -1 ========================================== + Hits 892 893 +1 Misses 38 38 + Partials 24 23 -1 ```

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