Enable module override when test.

[NaokiTsuchiya]

テスト実行時にモジュールをオーバライド可能にしました。

• overrideModule が指定されている場合は、Injectorを生成
• テストでは OverrideModule トレイトを使います。$this->app の型が親クラスに定義されており、上書きできないためこの方法にしています。

demoアプリに適用した際の差分は以下のとおりです。
https://github.com/koriym/hello-ray-di-for-laravel/compare/master...NaokiTsuchiya:feat/override-module?expand=1

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (2d97eec) 100.00% compared to head (c058571) 100.00%.

❗ Current head c058571 differs from pull request most recent head cb2fb4a. Consider uploading reports for the commit cb2fb4a to get more accurate results

Additional details and impacted files

@@             Coverage Diff             @@
##                 1.x       #12   +/-   ##
===========================================
  Coverage     100.00%   100.00%           
- Complexity        10        18    +8     
===========================================
  Files              2         3    +1     
  Lines             25        50   +25     
===========================================
+ Hits              25        50   +25     

Impacted Files	Coverage Δ
src/Application.php	100.00% <100.00%> (ø)
src/Testing/OverrideModule.php	100.00% <100.00%> (ø)

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