Fix running tests hanging indefinitely

By setting the `IsTestProject` property to false in Testcontainers.Commons.csproj

Cause: referencing the `xunit` package enables `IsTestProject=true` which trigger a bug in vstest where [netstandard libraries hangs indefinitely][1]. This bug was addressed 5 days ago: [Fix execution gets stucks on single netstandard source][2]

[1]: microsoft/vstest#4392
[2]: microsoft/vstest#4497

Author: 0xced

tests/Testcontainers.Commons/Testcontainers.Commons.csproj

@@ -3,6 +3,7 @@
         <TargetFrameworks>netstandard2.0;netstandard2.1</TargetFrameworks>
         <IsPackable>false</IsPackable>
         <IsPublishable>false</IsPublishable>
+        <IsTestProject>false</IsTestProject>
         <SignAssembly>true</SignAssembly>
         <LangVersion>latest</LangVersion>
     </PropertyGroup>