Fix For Xunit Async Tests (#3812)

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[Contribute](https://github.com/reactiveui/reactiveui#contribute)
section of the README -->

**What kind of change does this PR introduce?**
<!-- Bug fix, feature, docs update, ... -->

Fix for XUint tests

**What is the current behavior?**
<!-- You can also link to an open issue here. -->

Xunit 2.6.6 + fails to pass tests due to parallelism

**What is the new behavior?**
<!-- If this is a feature change -->

Using XUnit 2.8.0 with config file setting the parallelAlgorithm to
aggressive

**What might this PR break?**

none

**Please check if the PR fulfills these requirements**
- [ ] Tests for the changes have been added (for bug fixes / features)
- [ ] Docs have been added / updated (for bug fixes / features)

**Other information**:

src/Directory.Packages.props

@@ -7,8 +7,8 @@
     <FodyVersion>6.8.0</FodyVersion>
     <SplatVersion>15.0.1</SplatVersion>
     <XamarinAndroidSupportVersion>28.0.0.3</XamarinAndroidSupportVersion>
-    <XUnitVersion>2.6.6</XUnitVersion>
-    <VerifyXUnitVersion>23.2.0</VerifyXUnitVersion>
+    <XUnitVersion>2.8.0</XUnitVersion>
+    <VerifyXUnitVersion>24.2.0</VerifyXUnitVersion>
   </PropertyGroup>
   <ItemGroup>
     <PackageVersion Include="coverlet.msbuild" Version="6.0.2" />

src/ReactiveUI.Testing.Tests/ReactiveUI.Testing.Tests.csproj

@@ -12,4 +12,8 @@
   <ItemGroup>
     <None Include="API\**\*.txt" CopyToOutputDirectory="PreserveNewest" LinkBase="/" />
   </ItemGroup>
+
+  <ItemGroup>
+    <Content Include="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
+  </ItemGroup>
 </Project>

src/ReactiveUI.Testing.Tests/xunit.runner.json

@@ -0,0 +1,4 @@
+{
+    "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
+    "parallelAlgorithm": "aggressive"
+}

src/ReactiveUI.Tests/ReactiveUI.Tests.csproj

@@ -58,6 +58,10 @@
     <None Remove="Platforms\wpf\Mocks\FakeViewWithContractLocator3.cs" />
     <None Remove="Platforms\wpf\WpfGeneralFixture.cs" />
     <None Remove="Platforms\wpf\WpfGeneralFixtureApp.cs" />
+    <None Remove="xunit.runner.json" />
+  </ItemGroup>
+  <ItemGroup>
+    <Content Include="xunit.runner.json" CopyToOutputDirectory="PreserveNewest" />
   </ItemGroup>
   <ItemGroup>
     <Compile Update="Platforms\Winforms\Mocks\TestForm.Designer.cs">

src/ReactiveUI.Tests/xunit.runner.json

@@ -0,0 +1,4 @@
+{
+    "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
+    "parallelAlgorithm": "aggressive"
+}