Avoid failure of test_basic_report on 32 bit OS

report.generators.threading module uses sys._current_frames() to
get thread id, but it will get a negative number on 32 bit operating
system, which will cause test_guru_meditation_report.test_basic_report
fail since it expects a positive number.

This patch add an optional character '-' to pattern parameter of
re.match to avoid such problem.

Change-Id: If844894dc1f1032c78d6ed78d221aac27c42712e
Closes-Bug: #1264906
Merge "Avoid failure of test_basic_report on 32 bit OS"