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/prometheus-1.sh

@@ -1,7 +1,7 @@
 #!/bin/bash -e
 
 # run empty graph, with prometheus support
-timeout --kill-after=20s 15s ./mgmt run --tmp-prefix --prometheus &
+$timeout --kill-after=20s 15s ./mgmt run --tmp-prefix --prometheus &
 pid=$!
 sleep 5s	# let it converge