Cleanup 3

[zrzka]

• Not all tests were starting with the test_ prefix
• It's not necessary to check for is_ok() in tests, simple unwrap() is enough
• Some tests were rewritten
• TerminalCursor::reset_position() renamed to restore_position()
  • I read reset_position as a set it to (0, 0) or something like this.
  • restore is much more clear.
• Removed get_ prefixes from methods (not from functions)
• Renamed ScreenBuffer::create() -> new()