[tests] Fix flaky standalone/io/http_auth_test

This is an attempt to fix flaky failure

```
Unhandled exception:
Exception: Fatal: asyncStart() was called even though we are done with testing.. This is most likely a bug in your test.
#0      asyncStart (package:expect/async_helper.dart:53)
#1      asyncExpectThrows (package:expect/async_helper.dart:140)
#2      testMalformedAuthenticateHeaderWithAuthHandler.<anonymous closure> (file:///C:/b/s/w/ir/tests/standalone/io/http_auth_test.dart:263)
#3      Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:951)
#4      Future._propagateToListeners (dart:async/future_impl.dart:980)
#5      Future._completeWithValue (dart:async/future_impl.dart:723)
#6      Future._asyncCompleteWithValue.<anonymous closure> (dart:async/future_impl.dart:807)
#7      _microtaskLoop (dart:async/schedule_microtask.dart:42)
#8      _startMicrotaskLoop (dart:async/schedule_microtask.dart:60)
#9      _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:127)
#10     _RawReceivePort._handleMessage (dart:isolate-patch/isolate_patch.dart:194)

--- Re-run this test:
python3 tools/test.py -n vm-aot-win-release-x64 standalone/io/http_auth_test
```

The test has multiple calls to asyncExpectThrows which uses
asyncStart/asyncEnd, but only one top-level asyncStart/asyncEnd is
allowed.

In addition to a top-level asyncStart/asyncEnd, also add
asyncStart/asyncEnd around creation and shutdown of the server in order
to make sure all test cases are completed before test is finished.

TEST=standalone/io/http_auth_test

Change-Id: Ibb55dbae2c2e48e18f51f9a0adc24101d6121a8c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/425600
Reviewed-by: Brian Quinlan <[email protected]>
Commit-Queue: Alexander Markov <[email protected]>

Author: alexmarkov

tests/standalone/io/http_auth_test.dart

@@ -14,6 +14,7 @@ class Server {
   bool passwordChanged = false;
 
   Future<Server> start() {
+    asyncStart();
     var completer = new Completer<Server>();
     HttpServer.bind("127.0.0.1", 0).then((s) {
       server = s;
@@ -79,7 +80,9 @@ class Server {
   }
 
   void shutdown() {
-    server.close();
+    server.close().then((_) {
+      asyncEnd();
+    });
   }
 
   int get port => server.port;
@@ -362,6 +365,8 @@ void testLocalServerDigest() {
 }
 
 main() {
+  asyncStart();
+
   testUrlUserInfo();
   testBasicNoCredentials();
   testBasicCredentials();
@@ -374,4 +379,6 @@ main() {
   //testLocalServerBasic();
   //testLocalServerBearer();
   //testLocalServerDigest();
+
+  asyncEnd();
 }