Fix test group name for sound flow analysis.

I accidentally called the test group "Sound null safety", which is a
related concept, but not the same thing.

Sound null safety is a compilation mode in which no legacy code is
allowed, and so it is sound to assume that an expression with a
non-nullable type cannot evaluate to `null`. It has been the only
allowed way to compile Dart programs for some time.

Sound flow analysis, on the other hand, is a set of improvements to
flow analysis which are possible now that all Dart programs are now
compiled in sound null safety mode. It is a language-versioned feature
that is enabled in Dart 3.9.

Change-Id: Ib13d28be7fecea6d0e931b33d6643afab2349629
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/432004
Auto-Submit: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/test/flow_analysis/flow_analysis_test.dart

@@ -10646,7 +10646,7 @@ main() {
     });
   });
 
-  group('Sound null safety:', () {
+  group('Sound flow analysis:', () {
     group('<nonNull> as Null:', () {
       test('When enabled, is guaranteed to throw', () {
         h.run([expr('int').as_('Null'), checkReachable(false)]);