[vm_service] Rename another test to _test

I missed this one in
https://dart-review.googlesource.com/c/sdk/+/400320/ - although I did
add the entry for it in pkg/pkg.status :/

Change-Id: Iedcd0ba4804b136e73397939183a1535becec4fe
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/400560
Reviewed-by: Alexander Thomas <[email protected]>
Commit-Queue: Jens Johansen <[email protected]>

Author: jensjoha

pkg/vm_service/test/issue_59661.dart renamed to pkg/vm_service/test/issue_59661_test.dart

@@ -132,7 +132,7 @@ final tests = <IsolateTest>[
 void main([args = const <String>[]]) => runIsolateTests(
       args,
       tests,
-      'issue_59661.dart',
+      'issue_59661_test.dart',
       testeeConcurrent: code,
       pauseOnStart: false,
       pauseOnExit: true,