test_project.py: Remove unused variable in 'with' statement

flake8 reports:

 F841 local variable 'f' is assigned to but never used

Change-Id: If808eb381ee44c7da71e6281615a06a6723cf945
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/254593
Tested-by: David Pursehouse <dpursehouse@collab.net>
Reviewed-by: Mike Frysinger <vapier@google.com>