Interestingly, this allows me to reproduce #16603, while the previous
way hid the problem.

We explicitly document the Unsafe Browser as supported only in order
to connect to captive portals ⇒ let's not spend any time on testing
other use cases.

Here we want to check that no default password works. If one sets an
administration password, any such default password is replaced
by the one chosen by the user, so this test is moot.

Let's exercise functionality that is more likely to break
and is not exercised anywhere else in our manual & automated
test suites.

It's pretty hard to find SMTP relays that support delivery over cleartext SMTP
these days (thankfully). Let's focus on the most realistic option.

The corresponding automated tests have the check_tor_leaks tag so a proxy
and firewall bypass would be noticed.

Comparing the hashes is sufficient.

This sanity check was added after a RM or three made mistakes in the "Prepare
upgrade-description files" section, which was recently improved, so one
may hope that this check is not needed anymore.

kibi didn't understand at all these instructions, which suggests
they are not useful.

And anyway, worst case, if a UDF is missing, one of our automated test cases
will fail, which should alert us about the problem.