Bump NSubstitute from 4.3.0 to 4.4.0 (#994)

Bumps NSubstitute from 4.3.0 to 4.4.0.

---
updated-dependencies:
- dependency-name: NSubstitute
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

src/NUnitTestAdapterTests/NUnit.TestAdapter.Tests.csproj

@@ -13,7 +13,7 @@
   </PropertyGroup>
 
   <ItemGroup>
-    <PackageReference Include="NSubstitute" Version="4.3.0" />
+    <PackageReference Include="NSubstitute" Version="4.4.0" />
     <PackageReference Include="NUnit" Version="3.13.3" />
     <PackageReference Include="Microsoft.CodeAnalysis.CSharp" Version="2.10.0" />
     <PackageReference Include="NUnit.Analyzers" Version="3.3.0" />