[gdb/testsuite] Fix gdb.python/py-breakpoint.exp with python 2

[gdb/testsuite] Fix gdb.python/py-breakpoint.exp with python 2

With a gdb build using python 2.7, I run into:
...
(gdb) python \
  gdb.events.breakpoint_modified.connect(lambda bp: print(bp.enabled))^M
  File "<string>", line 1^M
    gdb.events.breakpoint_modified.connect(lambda bp: print(bp.enabled))^M
                                                          ^^M
SyntaxError: invalid syntax^M
Error while executing Python code.^M
(gdb) FAIL: gdb.python/py-breakpoint.exp: test_bkpt_auto_disable: \
  trap breakpoint_modified event
...

This is caused by the following:
- a lambda function body needs to be an expression
- in python 2, print is a statement, while in python 3 it's a function
- a function call is an expression, and a statement is not.

Fix this by defining a function print_bp_enabled:
...
def print_bp_enabled (bp):
    print (bp.enabled)
end
...
and using that instead.

Tested on x86_64-linux.