fix: Also bump System.Collections.Immutable in test project

Microsoft.NET.Test.Sdk 17.14.0 depends on System.Collections.Immutable version 8.0.0 or above.

```
Detected package downgrade: System.Collections.Immutable from 8.0.0 to 6.0.0.
Reference the package directly from the project to select a different version.  
...
nunit.analyzers.tests.csproj : error NU1605:  nunit.analyzers.tests ->
Microsoft.NET.Test.Sdk 17.14.0 -> Microsoft.TestPlatform.TestHost 17.14.0 ->
Microsoft.TestPlatform.ObjectModel 17.14.0 ->
System.Reflection.Metadata 8.0.0 -> System.Collections.Immutable (>= 8.0.0)
```

Author: mikkelbu

src/nunit.analyzers.tests/nunit.analyzers.tests.csproj

@@ -18,7 +18,7 @@
     <PackageReference Include="Gu.Roslyn.Asserts" Version="4.3.0" />
     <PackageReference Include="Gu.Roslyn.Asserts.Analyzers" Version="4.0.0" />
     <PackageReference Include="NUnit3TestAdapter" Version="5.0.0" />
-    <PackageReference Include="System.Collections.Immutable" Version="6.0.0" />
+    <PackageReference Include="System.Collections.Immutable" Version="8.0.0" />
   </ItemGroup>
 
   <ItemGroup Condition="'$(NUnitVersion)' == '4'">