Fix gdb.python/py-record-btrace-threads.exp with Python 3

Fix gdb.python/py-record-btrace-threads.exp with Python 3

Fix several instances of:

 ...
 python print not f1calls
   File "<string>", line 1
     print not f1calls
		     ^
 SyntaxError: Missing parentheses in call to 'print'
 Error while executing Python code.
 (gdb) FAIL: gdb.python/py-record-btrace-threads.exp: thread=1: checking thread 1: python print not f1calls
 ...

gdb/testsuite/ChangeLog:
2017-05-04  Pedro Alves  <palves@redhat.com>

	* gdb.python/py-record-btrace-threads.exp (check_insn_for_thread):
	Add parens to print call for Python 3.