harden HubSpec duration (#6030)

Needed to boost time to process 20,000 elements with buffer size =1 from 3 cumulative seconds to 300. On low quality AzDo machines there's no telling what delays we'll hit.

src/core/Akka.Streams.Tests/Dsl/HubSpec.cs

@@ -191,7 +191,7 @@ public async Task MergeHub_must_work_with_long_streams_when_buffer_size_is_1()
                 Source.From(Enumerable.Range(10001, 10000)).RunWith(sink, Materializer);
 
                 await probe.RequestAsync(int.MaxValue);
-                var result = await probe.ExpectNextNAsync(20000, 3.Seconds()).ToListAsync();
+                var result = await probe.ExpectNextNAsync(20000, 300.Seconds()).ToListAsync();
                 result.OrderBy(x => x).Should().BeEquivalentTo(Enumerable.Range(1, 20000));
             }, Materializer);
         }