ioutil package was deprecated in Go 1.16, replace all uses with their
respective replacements.
This patch was generated with a combination of `gofmt -r`, `eg`, and
manually (for `ioutil.ReadDir`).
Some tests require picking ports, and today resort to hard-coding,
which is brittle.
This patch adds a testlib.GetFreePort function to help pick free ports.
It is not totally race-free, but is much better than hard-coding.
This patch makes chasquid reload domaininfo periodically, so it notices
any external changes made to it.
It is in line with what we do for aliases and authentication already,
and makes it possible for external removals an additions to the
domaininfo database to be picked up without a restart.
We have many places in our tests where we create temporary directories,
which we later remove (most of the time). We have at least 3 helpers to
do this, and various places where it's done ad-hoc (and the cleanup is
not always present).
To try to reduce the clutter, and make the tests more uniform and
readable, this patch introduces two helpers in a new "testutil" package:
one for creating and one for removing temporary directories.
These new functions are safer, better tested, and make the tests more
consistent. All the tests are updated to use them.
