Disable building async tests on configurations that are not yet supported. (#116084)

Author: VSadov

src/tests/async/Directory.Build.targets

@@ -1,4 +1,9 @@
 <Project>
+  <PropertyGroup>
+    <!-- disable building for configurations that do not yet support runtime async. -->
+    <DisableProjectBuild Condition="'$(RuntimeFlavor)' == 'mono' or '$(TestBuildMode)' == 'nativeaot' or '$(TargetArchitecture)' == 'wasm'">true</DisableProjectBuild>
+  </PropertyGroup>
+
   <PropertyGroup>
     <!-- runtime async testing in main repo NYI -->
     <DisableProjectBuild>true</DisableProjectBuild>