Fixed tests in python/musexpr.py

If launched as __main__ python/musexpr.py had some troubles to run:

• several members of classes has been checked for existence by their name, but it should be done by hasatr()
• the base class Event didn't invoke super() of Music, still the last one introduces several important members
• expr.ly_expression() is used in test for print (line:2760), but not every kind of expression has this particular method, but <expr>.print_ly() used instead. As the last method definitely exists and invokes <expr>.ly_expression() if it needs — I used default Output_printer here.