Revert "Try using merged test runners"

This reverts commit 4ed06ec.

Author: sbomer

src/tests/nativeaot/Directory.Build.props

@@ -7,6 +7,5 @@
     <EnableAggressiveTrimming>true</EnableAggressiveTrimming>
     <!-- We need the library import generator when running android tests. -->
     <DisableImplicitFrameworkReferences>false</DisableImplicitFrameworkReferences>
-    <IsMergedTestRunnerAssembly>true</IsMergedTestRunnerAssembly>
   </PropertyGroup>
 </Project>