[TestBreakpointCommand] Fix after r251121

Summary:
"from __future__ import print_function" was added to the test file but
not to the embedded interpreter. This change uses file.write instead to
avoid all problems with print.

Reviewers: zturner

Subscribers: zturner, lldb-commits

Differential Revision: http://reviews.llvm.org/D14029

llvm-svn: 251150