Fix build and run CommandLine tests on all target frameworks.

Author: rstarkov

Tests/RT.CommandLine.Tests/RT.CommandLine.Tests.csproj

@@ -1,7 +1,7 @@
 <Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net7.0</TargetFrameworks>
+    <TargetFrameworks>net472;net5.0;net7.0</TargetFrameworks>
 
     <IsPackable>false</IsPackable>
   </PropertyGroup>