Fix terminal bug in WSL

Moved to shutil.get_terminal_size() from os.get_terminal_size() as:

• piping pyleet data through another program would crash on WSL using the os version, works with shutil
• shutil works while piping the output to a file instead of on the terminal, os doesn't.

More bugfixes

• Fix errors not being counted in failed test cases

Bugfixes

• A crash in a test no longer crashes the whole test suite.
• Modifications to the inputs in the validator function (for eg. in-place array modifications) no longer reflect in the console output.