chore: add setting to disable tfm parallel tests

Author: filzrev

src/BenchmarkDotNet.TestAdapter/build/BenchmarkDotNet.TestAdapter.props

@@ -1,6 +1,8 @@
 <Project InitialTargets="GenerateBDNEntryPoint" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
   <PropertyGroup>
     <EntryPointSourceDirectory>$(MSBuildThisFileDirectory)..\entrypoints\</EntryPointSourceDirectory>
+    <!-- Disable parallel tests between TargetFrameworks, if it's not explicitly specified. -->
+    <TestTfmsInParallel Condition="'$(TestTfmsInParallel)' = ''">true</TestTfmsInParallel>
   </PropertyGroup>
   <!-- 
     Microsoft.NET.Test.Sdk uses a property called "GenerateProgramFile" to determine whether it generates an entry