Using `get_stack()` after `yield()` results in a incomplete stack trace

[MikeSchulze]

Godot version: Godot Engine v3.2.3.stable.mono.official - https://godotengine.org OpenGL ES 3.0 Renderer: GeForce GTX 980/PCIe/SSE2

OS/device including version: Windows 10

Issue description: Using get_stack() after yield results in a incomplete stack and makes get_stack() unusless.

Steps to reproduce:

• create a func like this

  func test_get_stack_after_yield():
  prints("--Before--")
  print_stack()
  yield(get_tree().create_timer(0.100), "timeout")
  prints("--After--")
  print_stack()

  The output is

--Before-- Frame 0 - res://addons/gdUnit/test/monitor/PushErrorMonitorTest.gd:45 in function 'test_get_stack_after_yield' Frame 1 - res://addons/gdUnit/src/core/GdUnitExecutor.gd:155 in function 'execute_yielded' --After-- Frame 0 - res://addons/gdUnit/test/monitor/PushErrorMonitorTest.gd:48 in function 'test_get_stack_after_yield'

The second print_stack`()' misses the frame 1

**Minimal reproduction project:**
see *Steps to reproduce:*

[Calinou]

This is unlikely to be fixed before 4.0 is released since it removes yield in favor of await.

[dalexeev]

I'm not sure if this can be fully considered a bug. The point is, the function call resumed after yield() is actually detached from the main stack. The control flow is split into two parallel ones. Yes, this is inconvenient for debugging, but it reflects the reality.

[MikeSchulze]

@dalexeev sure yield is splitup, but for debugging reason it is very bad to miss the full stack trace ;)