feat: add WaitForMsg function

[Broderick-Westrope]

Closes #310. Please see the linked issue for more context on the goals of (and motivations for) this PR.
Duplicate of #311; I had some issues with GitHub account that deleted the forked repo :(

Changes:

• Rename existing WaitFor function to WaitForOutput to avoid confusion with the new, similarly named method.
• Add msgbuffer type. This is a simple mutex-protected slice of tea.Msg.
• Add msgCaptureModel type. Implements tea.Model. Contains a child tea.Model and a msgBuffer. Every tea.Msg is stored in the buffer before being sent to the child model.
• Changes to NewTestModel so that the provided model is now wrapped in a msgCaptureModel.
  • Perhaps this could be disabled by default an enabled with a TestOption.
• Add method TestModel WaitForMsg which takes a condition function and returns a tea.Msg. Much of the code for this was copied from the WaitFor function (now called WaitForOutput). At a set interval it will check the messages stored within the buffer and if any matches the provided condition function it will be returned.

Please note this is not a complete solution. Instead, this is intended to spark a discussion. I am happy to put in the required work to polish this off, but I want to make sure it is something that the Charm team wants in their repo before spending more time on it :)

[Broderick-Westrope]

@caarlos0 could I get re-approval on this? It's a duplicate of #311 which closed due to some issues with my GitHub account that are now resolved.

[Broderick-Westrope]

Hi @caarlos0, just bumping this one :)