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/t-02-exim/run.sh

@@ -26,6 +26,7 @@ set -e
 . $(dirname ${0})/../util/lib.sh
 
 init
+check_hostaliases
 
 if ! .exim4/exim4 --version > /dev/null; then
 	skip "exim4 binary at .exim4/exim4 is not functional"