Akka.TestKit: configurable `expect-no-message-default` value

[Aaronontheweb]

Is your feature request related to a problem? Please describe.

I believe we're missing a setting in the Akka.NET testkit for configuring ExpectNoMessage / ExpectNoMessageAsync timeouts independently from the ExpectMsg / ExpectMsgAsync ones:

akka.test{
  # Duration to wait in expectNoMessage by default.
  # Dilated by the timefactor.
  expect-no-message-default = 100ms
}

This setting is preferable 3s timeout we have currently (shorter test execution time and honestly, more useful.) However, the ability to have this setting be configurable separately from the expect-msg-default timeout would be very useful.

Describe the solution you'd like

So changing this value to 100ms by default would be a breaking behavioral change that would likely cause some problems for existing users of the software, not to mention our own test suite. Therefore, it's probably best to introduce the akka.test.expect-no-message-default and have it set to 3s - the same value that it is currently. We can change the default value in Akka.NET 1.6 or 2.0.

Doing it this way allows us to configure ExpectNoMessage / ExpectNoMessageAsync separately within an entire test fixture without changing the current values for all users.