fix: Add test for SCP (#2692)

* fix: Elongate agent disconnect timeout in tests

This will fix the flake seen here:
https://github.com/coder/coder/runs/7071719863?check_suite_focus=true

* fix: Add test for SCP

This was hanging due to the stdin pipe never being closed.
A test has been added to make sure it works!

agent/agent.go

@@ -467,6 +467,7 @@ func (a *agent) handleSSHSession(session ssh.Session) error {
 	}
 	go func() {
 		_, _ = io.Copy(stdinPipe, session)
+		_ = stdinPipe.Close()
 	}()
 	err = cmd.Start()
 	if err != nil {