[lldb] Replace TestAbortExitCode with a debugserver specific test

When I added TestAbortExitCode I actually planned this to be a generic test for the
exit code functionality on POSIX systems. However due to all the different test setups we
can have I don't think this worked out. Right now the test had to be made so permissive
that it pretty much can't fail.

Just to summarize, we would need to support the following situations:
1. ToT debugserver (on macOS)
2. lldb-server (on other platforms)
3. Any old debugserver version when using the system debugserver (on macOS)

This patch is removing TestAbortExitCode and adds a ToT debugserver specific test
that checks the patch that motivated the whole exit code testing. There is already
an exit-code test for lldb-server from what I can see and 3) is pretty much untestable
as we don't know anything about the system debugserver.

Reviewed By: kastiglione

Differential Revision: https://reviews.llvm.org/D89305