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/t5.sh

@@ -1,7 +1,7 @@
 #!/bin/bash -e
 
 # should take slightly more than 35s, but fail if we take 45s)
-timeout --kill-after=45s 40s ./mgmt run --yaml t5.yaml --converged-timeout=5 --no-watch --tmp-prefix &
+$timeout --kill-after=45s 40s ./mgmt run --yaml t5.yaml --converged-timeout=5 --no-watch --tmp-prefix &
 pid=$!
 wait $pid	# get exit status
 exit $?