Add two examples of custom lua cpu tests

src/lua/empty-test.lua

@@ -0,0 +1,24 @@
+#!/usr/bin/env sysbench
+
+-- you can run this script like this:
+-- $ ./empty-test.lua --cpu-max-prime=20000 --threads=8 --histogram --report-interval=1 run
+
+sysbench.cmdline.options = {
+    -- the default values for built-in options are currently ignored, see 
+    -- https://github.com/akopytov/sysbench/issues/151
+    ["cpu-max-prime"] = {"CPU maximum prime", 10000},
+    ["threads"] = {"Number of threads", 2},
+    ["histogram"] = {"Show histogram", "on"},
+    ["report-interval"] = {"Report interval", 1}
+}
+
+function event()
+end
+
+function sysbench.hooks.report_intermediate(stat)
+    local seconds = stat.time_interval
+    print(string.format("%.0f;%u;%4.2f",
+        stat.time_total, 
+        stat.threads_running,
+        stat.events / seconds))
+end