Download Microsoft.TestPlatform.Portable

To workaround bug in Stryker.NET 4.4.0 that was fixed by stryker-mutator/stryker-net#3105

Author: 0xced

tests/Serilog.Formatting.Log4Net.Tests.csproj

@@ -21,6 +21,10 @@
     <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2" PrivateAssets="all" />
   </ItemGroup>
 
+  <ItemGroup>
+    <PackageDownload Include="Microsoft.TestPlatform.Portable" version="[17.11.1]" />
+  </ItemGroup>
+
   <ItemGroup>
     <VSTestLogger Include="GitHubActions%3Bsummary.includePassedTests=true%3Bsummary.includeSkippedTests=true" Visible="false" Condition="$(GITHUB_STEP_SUMMARY) != ''" />
     <VSTestLogger Include="trx%3BLogFileName=TestResults-$(TargetFramework).trx" Visible="false" Condition="$(ContinuousIntegrationBuild) == 'true'" />