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/util.sh

@@ -1,5 +1,11 @@
 # common settings and functions for test scripts
 
+if [[ $(uname) == "Darwin" ]] ; then
+	export timeout="gtimeout"
+else
+	export timeout="timeout"
+fi
+
 fail_test()
 {
 	echo "FAIL: $@"