The "close the Unsafe Browser" without a "I" was a very ad-hoc optimization,
that will break on next change. Stay tuned.

This avoids using a command to run it in the test suite that is different from
the command used to run it from the Applications menu, as was the case
previously. This also avoids hardcoding information that may change.

And, by necessity, catch more kinds of sudo error messages.

... especially when the problem is supposed to be fixed already:
we do want to make sure it is, and that no regression occurs.

This includes some guidelines for writing tests.

This reverts commit acd93758.

syslinux is already listed on the above line.

It was pulled for liveusb-creator, that does not depend on it anymore.

See recent commit with same commit subject.

Instead we let the test suite use its default (which is the same as
what --newest-iso was supposed to do) when no --iso is given.

Otherwise these tests fail for me.