[test] Explicitly pass sound mode flag

Change-Id: I1e6f7fbc96cddbb8a98a5a3f32bfeab792ce1bbb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/277041
Reviewed-by: Srujan Gaddam <[email protected]>
Commit-Queue: Nicholas Shahan <[email protected]>

Author: nshahan

pkg/compiler/tool/kernel_visitor/test/info_visitor_test.dart

@@ -89,6 +89,7 @@ void main() async {
   var result = await Process.run(Platform.resolvedExecutable, [
     compilePath,
     "--target=dartdevc",
+    "--nnbd-strong",
     "--platform=${ddcOutlinePath}",
     "-o=${dillPath}",
     testClassesPath