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/src/Plugin.cc

@@ -0,0 +1,69 @@
+
+#include "Plugin.h"
+
+namespace zeek::run_state
+	{
+extern double processing_start_time;
+	}
+
+namespace btest::plugin::Demo_Iosource
+	{
+Plugin plugin;
+	}
+
+using namespace btest::plugin::Demo_Iosource;
+
+zeek::plugin::Configuration Plugin::Configure()
+	{
+	EnableHook(zeek::plugin::HOOK_DRAIN_EVENTS, 0);
+
+	zeek::plugin::Configuration config;
+	config.name = "Demo::Iosource";
+	config.description = "This is a iosource";
+	config.version.major = 1;
+	config.version.minor = 0;
+	config.version.patch = 0;
+	return config;
+	}
+
+void Plugin::InitPostScript()
+	{
+	std::fprintf(stdout, "%.6f InitPostScript\n", zeek::run_state::network_time);
+	ts1 = new TimeoutSource("timeout-source-1");
+	ts2 = new TimeoutSource("timeout-source-2");
+	fd1 = new FdSource("fd-source-1");
+	fd2 = new FdSource("fd-source-2");
+	}
+
+void Plugin::HookDrainEvents()
+	{
+	++round;
+	if ( zeek::run_state::processing_start_time != 0.0 ) // ignore drains from dispatch_packet
+		return;
+	//
+	std::fprintf(stdout, "%.6f HookDrainEvents %d\n", zeek::run_state::network_time, round);
+
+	if ( (round % 9) == 0 )
+		{
+		std::fprintf(stdout, "%.6f   Firing %s\n", zeek::run_state::network_time, ts1->Tag());
+		ts1->Fire();
+		}
+
+	if ( (round % 19) == 0 )
+		{
+		std::fprintf(stdout, "%.6f   Firing %s\n", zeek::run_state::network_time, ts2->Tag());
+		ts2->Fire();
+		}
+
+	if ( (round % 19) == 0 )
+		{
+		std::fprintf(stdout, "%.6f   Firing %s\n", zeek::run_state::network_time, fd1->Tag());
+		fd1->Fire();
+		}
+
+	if ( (round % 23) == 0 )
+		{
+		std::fprintf(stdout, "%.6f   Firing %s\n", zeek::run_state::network_time, fd2->Tag());
+		fd2->Fire();
+		}
+	}