## Summary of the Pull Request
This PR implements a pair of shims for `cmd` and `powershell`, so that their `cls` and `Clear-Host` functions will clear the entire terminal buffer (like they do in conhost), instead of just the viewport. With the conpty viewport and buffer being the same size, there's effectively no way to know if an application is calling these API's in this way with the intention of clearing the buffer or the viewport. We absolutely have to guess.
Each of these shims checks to see if the way that the API is being called exactly matches the way `cmd` or `powershell` would call these APIs. If it does, we manually write a `^[[3J` to the connected terminal, to get he Terminal to clear it's own scrollback.
~~_⚠️ If another application were trying to clear the **viewport** with an exactly similar API call, this would also cause the terminal scrollback to get cleared ⚠️_~~
* [x] Should these shims be restricted to when the process that's calling them is actually `cmd.exe` or `powershell.exe`? Can I even do this? I think we've done such a good job of isolating the client process information from the rest of the host code that I can't figure out how to do this.
- YES, this can be done, and I did it.
* [ ] **TODO**: _While I'm here_, should I have `DoSrvPrivateEraseAll` (the implementation for `^[[2J`, in `getset.cpp`) also manually trigger a EraseAll in the terminal in conpty mode?
## PR Checklist
* [x] Closes#3126
* [x] Actually closes#1305 too, which is really the same thing, but probably deserves a callout
* [x] I work here
* [x] Tests added/passed
* [n/a] Requires documentation to be updated
## Validation Steps Performed
* ran tests
* checked `cls` in the Terminal
* checked `Clear-Host` in the Terminal
* Checked running `powershell clear-host` from `cmd.exe`
## Summary of the Pull Request
#4354 is a pretty complicated PR. It's got a bunch of conpty changes, but what it also has was some critical improvements to the roundtrip test suite. I'm working on some other bugfixes in the same area currently, and need these tests enhancements in those branches _now_. The rest of #4354 is complex enough that I don't trust it will get merged soon (if ever). However, these fixes _should_ be in regardless.
## PR Checklist
* [x] Taken directly from #4354
* [x] I work here
* [x] Tests added/passed
* [n/a] Requires documentation to be updated
## Detailed Description of the Pull Request / Additional comments
This is four main changes:
* Enable conpty to be fully enabled in unittests. Just setting up a VT renderer isn't enough to trick the host into being in conpty mode - it also needs to have some other flags set.
* Some minor changes to `CommonState` to better configure the common test state for conpty
* Move some of the verify helpers from `ConptyRoundtripTests` into their own helper class, to be shared in multiple tests
* Add a `TerminalBufferTests` class, for testing the Terminal buffer directly (without conpty).
This change is really easier than
would suggest, I promise.
