[vm/shared/test] Add Encoding.getByName test when used in isolategroup-bound context.

Follow-up to https://dart-review.googlesource.com/c/sdk/+/461185/comment/323d39ca_c22b0862/

TEST=ci

Change-Id: Iea9c68c0761a63804e87d9d2f54f1571ff983cd9
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/465962
Commit-Queue: Alexander Aprelev <[email protected]>
Reviewed-by: Lasse Nielsen <[email protected]>

Author: aam

tests/ffi/run_isolate_group_run_test.dart

@@ -278,6 +278,14 @@ main(List<String> args) {
       () => Encoding.getByName("us-ascii")!.encode("123abc"),
     ),
   );
+  Expect.identical(
+    ascii,
+    IsolateGroup.runSync(() => Encoding.getByName("us-ascii")),
+  );
+  Expect.identical(
+    utf8,
+    IsolateGroup.runSync(() => Encoding.getByName("utf-8")),
+  );
 
   print("All tests completed :)");
 }