Optionally record exit status to a special expected output file

[sampsyo]

Currently, to specify an expected nonzero exit status for a test command, you have to configure it manually: either in turnt.toml or in an embedded marker in the test. This works fine when, for example, handcrafting test programs with expected failures.

But for differential testing (a newish use case for Turnt that is still not completely seamless), this is annoying. We instead want to save the expected exit status from one command run, in a file, and then compare it against the exit status from another command—just like we do with textual output streams.

Maybe we can hack this in with another special output value in the config, just like - and 2 indicate the stdout and stderr streams. Like this:

output.status = '?'

That would tell Turnt to save the exit status in a file called <testname>.status.