[TESTING] Add unit tests for keymapping

[daandj]

❗ https://github.com/Felienne/hedy/issues/3337 has to be completed first.

Issue 3337 adds the keyword 'pressed' to the Hedy language to make key mappings possible. For example, the user could write something like the following:

if pressed is 'a' print 'cool!' else print 'meh'

To make sure that this works unit tests should be added that check if the correct Python code is generated.