Revert "Wait for commandline after killing (#19)"

[fkorotkov]

The latest changes were failing integration tests of Cirrus CI. Reverted and added a test.

Probably need reconsider #16

[edigaryev]

It seems to be working right, but due to this goroutine:

cirrus-ci-agent/internal/executor/shell.go

Lines 45 to 49 in f38456a

	go func() {
	// give time to flush logs
	time.Sleep(1 * time.Second)
	done <- cmd.Wait()
	}()

The Wait() is almost always called twice here:

cirrus-ci-agent/internal/executor/shell.go

Lines 71 to 74 in f38456a

	err = cmd.Wait()
	if err != nil {
	handler([]byte(fmt.Sprintf("\nFailed to wait for killed shell: %s", err)))
	}

Which triggers a sanity check inside Wait() (https://github.com/golang/go/blob/78c20c81aaa7d8d8a4aa2132806f51fb68f33a1e/src/os/exec/exec.go#L502-L505) and results in an unexpected logger output in tests.

I've created #21 to fix this.