Remove API baselines for M.A.Testing (#28258)

* Remove API baselines for M.A.Testing

* Just disable API baseline checks instead of setting it as test asset

Author: John Luo

src/Testing/src/Microsoft.AspNetCore.Testing.csproj

@@ -13,6 +13,8 @@
     <GenerateFrameworkReferenceAssembly>true</GenerateFrameworkReferenceAssembly>
     <!-- This package is internal, so we don't generate a package baseline. Always build against the latest dependencies. -->
     <UseLatestPackageReferences>true</UseLatestPackageReferences>
+    <!-- No need to track public APIs in test utilities. -->
+    <AddPublicApiAnalyzers>false</AddPublicApiAnalyzers>
   </PropertyGroup>
 
   <ItemGroup>