tests: use sha256line.py instead of /dev/random in test-censor.t (issue6858)

Sometimes the systems that run our test suite don't have enough entropy and
they cannot produce target file of the expected size using /dev/random, which
results in test failures. Switching to /dev/urandom would give us way more
available data at the cost of it being less "random", but we don't really need
to use entropy for this task at all, since we only care if the file size after
compression is big enough to not be stored inline in the revlog. So let's use
something that we already have used to generate this kind of data in other
tests.