Test output expectations should fail if the process has prematurely exited.

[mitchell-as]

The idea is to call exec.Cmd.Wait() in a goroutine (https://github.com/ActiveState/termtest/pull/12/files#diff-13ff3d9e77d1ebdf70d066552cb9e504472550dc97fd74aac53a8ab3a63e54a1) and edit the Expect() functions (https://github.com/ActiveState/termtest/pull/12/files#diff-a63ecf8315eeb0943ebec69499243bd650ae6b99635bde7aa1c0f6ac773368c2R106-R111 and https://github.com/ActiveState/termtest/pull/12/files#diff-2a74ac0f372490e4a5cf19a11e38b9329713643dc44c65ca5f7de285f84c9391R183-R187) to poll for a command's exit status alongside the usual expect checks. A helper function that returns a channel is used to facilitate use inside select{} statements (https://github.com/ActiveState/termtest/pull/12/files#diff-296a01a801f363652ec765418086e861f5db0287df573d54c8fde4eebe0e26e9R43).

https://github.com/ActiveState/cli/pull/3309 uses this PR's commit when running integration tests for State Tool to verify that it doesn't break anything at least.