[71-74] FileSinkSpec (#6619)

* [71-74] `FileSinkSpec`

* Revert "[71-74] `FileSinkSpec`"

This reverts commit 6ef392b.

* fix

---------

Co-authored-by: Aaron Stannard <aaron@petabridge.com>

src/core/Akka.Streams.Tests/IO/FileSinkSpec.cs

@@ -408,9 +408,9 @@ await AwaitAssertAsync(() =>
         }
 
         [Fact]
-        public void SynchronousFileSink_should_write_buffered_element_if_manual_flush_is_called()
+        public async Task SynchronousFileSink_should_write_buffered_element_if_manual_flush_is_called()
         {
-            this.AssertAllStagesStopped(async() => 
+            await this.AssertAllStagesStoppedAsync(async() => 
             {
                 await TargetFileAsync(async f =>
                 {
@@ -468,7 +468,7 @@ private async Task TargetFileAsync(
             {
                 // this is the proverbial stream kill switch, make sure that all streams
                 // are dead so that the file handle would be released
-                await this.AssertAllStagesStoppedAsync(async() => { }, materializer);
+                await this.AssertAllStagesStoppedAsync(() => { return Task.CompletedTask; }, materializer);
 
                 //give the system enough time to shutdown and release the file handle
                 await Task.Delay(500);