Fixed local errors in .NET Core 2.1 unit tests (#3365)

## PR Type
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 - Bugfix
 - Build or CI related changes
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## What is the current behavior?
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Some .NET Core 2.1 unit tests for the base package fail when run locally.

## What is the new behavior?
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All tests run correctly when executed locally.

## PR Checklist

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- [X] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] ~~Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->~~
- [ ] ~~Sample in sample app has been added / updated (for bug fixes / features)~~
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- [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [X] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [X] Contains **NO** breaking changes

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Author: msftbot[bot]

UnitTests/UnitTests.NetCore/UnitTests.NetCore.csproj

@@ -12,6 +12,11 @@
     <PackageReference Include="MSTest.TestFramework" Version="2.1.0" />
   </ItemGroup>
 
+  <!-- Workaround for the .NET Core 2.1 binary not resolving the Unsafe assembly properly -->
+  <ItemGroup Condition=" '$(TargetFramework)' == 'netcoreapp2.1' ">
+    <PackageReference Include="System.Runtime.CompilerServices.Unsafe" Version="4.7.1" />
+  </ItemGroup>
+
   <ItemGroup>
     <ProjectReference Include="..\..\Microsoft.Toolkit\Microsoft.Toolkit.csproj" />
   </ItemGroup>