[dart/vm] Reduce trip count of osr test

Rationale:
Test was borderline timeout. The reduced tripcount
still triggers OSR consistently, but runs a lot faster.

#36421
#36218
#36214

Change-Id: I60917bb6bdd51dee6bd11d1fe5beede4753c2147
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/99783
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Aart Bik <ajcbik@google.com>

tests/language_2/vm/osr_nonempty_stack_test.dart

@@ -98,7 +98,7 @@ List<int> test5(int m, int n) {
 }
 
 main() {
-  int n = 100000;
+  int n = 20000;
   int g = 457;
 
   var a = test1(n);