Add functional tests for application host

Follow @davidfowl 's suggestion here: https://github.com/aspnet/KRuntime/pull/808

add functional tests for `k` command to prevent any further regression.
It's only the header of help information, because verifying header of help information is enough here. The header doesn't change frequently. Moreover, help information header is not a constant that can be shared, it's dynamically generated:
https://github.com/aspnet/KRuntime/blob/dev/src/Microsoft.Framework.CommandLineUtils/CommandLine/CommandLineApplication.cs#L311