test: Enable macOS shell testing

* Check and install libvirt with Homebrew

  macOS does not have apt, dnf or yum. Add checking for homebrew for
  installing libvirt.

* Use platform timeout for tests
    * Add timeout detection to test/util.sh
    * Use $timeout for shell test requiring timeout

test/shell/t6.sh

@@ -7,7 +7,7 @@ if env | grep -q -e '^TRAVIS=true$'; then
 fi
 
 # run till completion
-timeout --kill-after=20s 15s ./mgmt run --yaml t6.yaml --no-watch --tmp-prefix &
+$timeout --kill-after=20s 15s ./mgmt run --yaml t6.yaml --no-watch --tmp-prefix &
 pid=$!
 sleep 1s	# let it converge
 test -e /tmp/mgmt/f1