tests: fix init subcmd after url change

My recent 401c6f07 ("init: make
--manifest-url flag optional") commit broke the unittest.

Change-Id: I19ad0e8c8cbb84ab5474ebc370e00acfe957e136
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/298223


Reviewed-by: Raman Tenneti <rtenneti@google.com>
Tested-by: Mike Frysinger <vapier@google.com>