Add more explicit nulls flag

Author: noti0na1

compiler/test/dotty/tools/vulpix/TestConfiguration.scala

@@ -66,7 +66,7 @@ object TestConfiguration {
   val defaultOptions = TestFlags(basicClasspath, commonOptions)
   val unindentOptions = TestFlags(basicClasspath, Array("-no-indent") ++ checkOptions ++ noCheckOptions ++ yCheckOptions)
   val withCompilerOptions =
-    defaultOptions.withClasspath(withCompilerClasspath).withRunClasspath(withCompilerClasspath)
+    defaultOptions.withClasspath(withCompilerClasspath).withRunClasspath(withCompilerClasspath) and "-Yexplicit-nulls"
   lazy val withStagingOptions =
     defaultOptions.withClasspath(withStagingClasspath).withRunClasspath(withStagingClasspath)
   lazy val withTastyInspectorOptions =