Add overridden duration timeout to StreamTestKit

[mdedetrich]

The context of this change is apache/pekko-http#590 (review). It turns out its extremely cumbersome to manually override the duration so I am adding this public method

@raboof @jrudolph Let me know if this line of thinking is on the right lines.

[mdedetrich]

There is code duplication here, I can clean it up if the premise of this PR is acceptable.

[mdedetrich]

This might look a bit un-orthodox by Pekko standards, but it is the idiomatic way to override a configuration when in ScalaTest (which is what StreamSpec extends).

The defaault behaviour is still the same (i.e. getting the value from pekko.stream.testkit.all-stages-stopped-timeout) but if you decide to override it then you can do it the ScalaTest specific way.

[pjfanning]

including in 1.1.2 milestone - if 1.1.1-RC1 vote fails, we can consider adding this to a 1.1.1-RC2

[mdedetrich]

Adding some reviewers to this PR to get an idea whether its even on the right track, @jrudolph @raboof Maybe you have some comments?

[mdedetrich]

The premise here is that by default we get the timeout value from Pekko typesafe config (as is expected when using Pekko) however if a user wants to override this then its done the ScalaTest idiomatic way (which is overriding a def in a trait, similar to how PatienceConfig works).

[jxnu-liguobin]

It is possible to omit units and support more flexibility, but the code is slightly complex and for reference only.

import scala.jdk.DurationConverters._
import org.scalatest.time.Span._
system.settings.config.getDuration("pekko.stream.testkit.all-stages-stopped-timeout").toScala.convertDurationToSpan

I just saw this {} and wanted to make some changes. I looked at the Span code and found that it has convertDurationoSpan. You can do whatever you want.