fix util.py/FileLock and lock_test

Do not pass os.O_TRUNC to open(), that immediately empties out the lock file.

Fix lock_test to be able to catch this error. So far things were happening too
fast to notice it.

Caught by running 'make check' on a main unit APU that has different timing
behavior.

Change-Id: I872a3d2548ac84097ac7acf13cb12c36822e076e
4 files changed