React to SIGTERM exit code change

[Tratcher]

dotnet/aspnetcore#6526
This required copying quite a bit of functional test infrastructure, though I was able to simplify it quite a bit. We may be able to refactor these so the ones in aspnetcore derive from these, but I don't know if it's worth it.

[Tratcher]

@tmds Generic Host is structured a bit differently and the lifetime doesn't have as clear a picture on if the application is exiting gracefully or not. This change suppresses the 143 exit code and does not appear to affect the exit code for unhandled exceptions.

[tmds]

This change suppresses the 143 exit code and does not appear to affect the exit code for unhandled exceptions.

coreclr change was on SIGTERM to have 143 exit code by default.
coreclr did not change regarding unhandled exceptions and still expects the application to take care of that.

fyi, System.CommandLine library catches unhandled exceptions and turns those into ExitCode 1: https://github.com/dotnet/command-line-api/blob/979e30b80482872354e279c67f79d3e94c11380a/src/System.CommandLine/Invocation/InvocationExtensions.cs#L134-L164.

[Tratcher]

bump

[jkotalik]

I'm fine with copying files for now, but this division in hosting between AspNetCore/Extensions isn't fun.

[JunTaoLuo]

I'm not a fan of duplicating the code everywhere especially since now the types live in both namespaces. If this is only needed for testing, is it possible to move all the integration test code to the test project and make it internal there? If the intention is to ship this as part of extensions, I'd like to think a bit more about how support the type in both extensions and aspnetcore.

[JunTaoLuo]

two minor outstanding comments but otherwise this looks good.

Cleaned up