Flow analysis: Fix test function name.

In a previous commit, I accidentally named these test functions
`test`.  Replace with a more descriptive name.

Bug: #44898
Change-Id: I662b6003a893dbb05fea88000f39e4e56b0cb725
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/196107
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: stereotype441

pkg/_fe_analyzer_shared/test/flow_analysis/why_not_promoted/data/argument_type_not_assignable_nullability_error.dart

@@ -446,7 +446,7 @@ class C33 {
   int? bad;
 }
 
-test(C33 c) {
+explicitExtensionInvocation(C33 c) {
   if (c.bad == null) return;
   E33(c.
           /*notPromoted(propertyNotPromoted(target: member:C33.bad, type: int?))*/ bad)

tests/language/why_not_promoted/argument_type_not_assignable_nullability_error_test.dart

@@ -655,7 +655,7 @@ class C33 {
   // [context 94] 'bad' refers to a property so it couldn't be promoted.
 }
 
-test(C33 c) {
+explicitExtensionInvocation(C33 c) {
   if (c.bad == null) return;
   E33(c.bad).f();
   //  ^^^^^