test: Skip integration tests if $HOSTALIASES is not functional

Most integration tests depend on the $HOSTALIASES environment variable
being functional. That variable works on most systems, but not all. In
particular, systems with `systemd-resolved` can cause the variable to be
ignored.

This was reported by Alex Ellwein in
https://github.com/albertito/chasquid/issues/20.

This patch makes the affected tests to be skipped if $HOSTALIASES is not
working properly. It also removes unnecessary hosts files from tests
which don't need it, and documents this behaviour.

Thanks to Alex Ellwein and foxcpp@ for reporting and helping investigate
this issue!

test/util/lib.sh

@@ -146,6 +146,12 @@ function fail() {
 	exit 1
 }
 
+function check_hostaliases() {
+	if ! "${UTILDIR}/check-hostaliases"; then
+		skip '$HOSTALIASES not working (probably systemd-resolved)'
+	fi
+}
+
 # Wait until there's something listening on the given port.
 function wait_until_ready() {
 	PORT=$1