tests/frontend/push.py: Fix test fallout from recent changes
This test was broken from the following commits:
98c807002cf3beb2110695083450a42fe8feefd0
9a124386a0ba8995e7cfd92e2c7d8fb23854f7e4
As of these commits, a Cli() instance requires it's directory to not be created in advance, this was not caught in CI because we skip the tests in CI where the runners lack proper subsecond mtime precision.