Add a new plugin test with verbose IO source output

This is mostly for documentation/verification purposes of how the IO loop
currently does draining and when it picks up FD based (non packet) IO
source. For example, it shows that currently FD based sources are processed
fairly delayed and that we now also process two timeout sources that are ready.

testing/btest/plugins/iosource-plugin.zeek

@@ -0,0 +1,45 @@
+# @TEST-EXEC: ${DIST}/auxil/zeek-aux/plugin-support/init-plugin -u . Demo Iosource
+# @TEST-EXEC: cp -r %DIR/iosource-plugin/* .
+
+# @TEST-EXEC: ./configure --zeek-dist=${DIST} && make
+#
+# @TEST-EXEC: ZEEK_PLUGIN_PATH=`pwd` zeek -Bmain-loop -b %INPUT -r $TRACES/wikipedia.trace > output
+# @TEST-EXEC: TEST_DIFF_CANONIFIER= btest-diff output
+
+@load-plugin Demo::Iosource
+
+global flushes = 0;
+global packets = 0;
+
+# Default is 100 for pcaps, but that only triggers a single Poll() when
+# reading wikipedia.trace. Tune it down a bit so Process on the FdSources
+# is called more often.
+redef io_poll_interval_default = 10;
+
+event zeek_init() {
+	print network_time(), "zeek_init";
+}
+
+event network_time_init() {
+	print network_time(), "network_time_init";
+}
+
+event raw_packet(p: raw_pkt_hdr)
+	{
+	++packets;
+	print network_time(), "raw_packet", packets;
+	}
+
+event event_queue_flush_point() {
+	++flushes;
+	print network_time(), "event_queue_flush_point", flushes;
+}
+
+event net_done(ts: time) {
+	print network_time(), "net_done", ts;
+}
+
+event zeek_done() {
+	print network_time(), "zeek_done";
+	print network_time(), "flushes", flushes, "packets", packets;
+}