Disable translations by default in our fake ttys

Disable echo, canonical mode, and line ending translation by default in our
ptys to fake TTY devices. That's usually what we want to simulate devices, and
relevant client software like modem-manager sets up the terminal anyway.
Add new method umockdev_testbed_load_script()

This loads a script file into the testbed, and starts a thread to replay
read()s and write()s from/to the device. Currently this does not allow any
deviation from the recorded script.