Adds additional tests for the interface. Creates a new base test class
GodotXtermTest that adds some additional assert methods. Tests
inheriting from this should override the got_described_class() method.
Add instance of the described class named 'subject' will be created and
added to the scene tree before each test.
Adds the option `use_threads` to PTY (enabled by default) which improves
performance when enabled. For example, running `time cat file.txt` where
file is ~4.5MB will take ~0.250s with threads enabled, versus >20s when
disabled.
