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.h

@@ -0,0 +1,83 @@
+
+#pragma once
+#include <zeek/Flare.h>
+#include <zeek/plugin/Plugin.h>
+#include <string>
+#include <string_view>
+
+#include "zeek/iosource/Manager.h"
+
+namespace btest::plugin::Demo_Iosource
+	{
+
+class TimeoutSource : public zeek::iosource::IOSource
+	{
+public:
+	TimeoutSource(std::string_view ident) : ident(ident)
+		{
+		zeek::iosource_mgr->Register(this, true /* don't count */);
+		};
+
+	void Process() override
+		{
+		std::fprintf(stdout, "%.3f %s TimeoutSource.Process()\n", zeek::run_state::network_time,
+		             ident.c_str());
+		on = false;
+		}
+
+	double GetNextTimeout() override { return on ? 0.0 : 0.1; };
+
+	const char* Tag() override { return ident.c_str(); };
+
+	void Fire() { on = true; }
+
+private:
+	std::string ident;
+	bool on = false;
+	};
+
+class FdSource : public zeek::iosource::IOSource
+	{
+public:
+	FdSource(std::string_view ident) : ident(ident)
+		{
+		zeek::iosource_mgr->Register(this, true /* don't count */);
+		if ( ! zeek::iosource_mgr->RegisterFd(flare.FD(), this) )
+			zeek::reporter->FatalError("Failed to register flare FD");
+		}
+
+	void Process() override
+		{
+		std::fprintf(stdout, "%.3f %s FdSource.Process()\n", zeek::run_state::network_time,
+		             ident.c_str());
+		flare.Extinguish();
+		}
+
+	double GetNextTimeout() override { return -1; }
+
+	const char* Tag() override { return ident.c_str(); };
+
+	void Fire() { flare.Fire(); }
+
+private:
+	std::string ident;
+	zeek::detail::Flare flare;
+	};
+
+class Plugin : public zeek::plugin::Plugin
+	{
+protected:
+	zeek::plugin::Configuration Configure() override;
+	void InitPostScript() override;
+	void HookDrainEvents() override;
+
+private:
+	int round = 0;
+	TimeoutSource* ts1 = nullptr;
+	TimeoutSource* ts2 = nullptr;
+	FdSource* fd1 = nullptr;
+	FdSource* fd2 = nullptr;
+	};
+
+extern Plugin plugin;
+	}