Bump Microsoft.NET.Test.Sdk from 16.8.0 to 16.9.1 (#128)

Author: dependabot-preview[bot]

src/common.props

@@ -12,7 +12,7 @@
   </PropertyGroup>
   <PropertyGroup>
     <XunitVersion>2.4.1</XunitVersion>
-    <TestSdkVersion>16.8.0</TestSdkVersion>
+    <TestSdkVersion>16.9.1</TestSdkVersion>
     <NBenchVersion>2.0.1</NBenchVersion>
     <AkkaVersion>1.4.16</AkkaVersion>
     <NetCoreTestVersion>netcoreapp3.1</NetCoreTestVersion>