Bump MSTest.TestFramework from 1.2.1 to 1.4.0

Bumps [MSTest.TestFramework](https://github.com/microsoft/testfx) from 1.2.1 to 1.4.0.
- [Release notes](https://github.com/microsoft/testfx/releases)
- [Commits](microsoft/testfx@v1.2.1...1.4.0)

Author: dependabot-preview[bot]

MyNumberNET_Test/MyNumberNET_Test.csproj

@@ -19,7 +19,7 @@
   <ItemGroup>
     <PackageReference Include="Microsoft.NET.Test.Sdk" Version="15.7.0" />
     <PackageReference Include="MSTest.TestAdapter" Version="1.2.1" />
-    <PackageReference Include="MSTest.TestFramework" Version="1.2.1" />
+    <PackageReference Include="MSTest.TestFramework" Version="1.4.0" />
   </ItemGroup>
   <ItemGroup>
     <ProjectReference Include="..\MyNumberNET\MyNumberNET.csproj" />