Add test creating multiple plugins with load dependencies.

If we load plugins purely alphabetically, the 1st Zeek run in the test
will success while the 2nd will fail.

testing/btest/plugins/plugin-load-dependency/2/src/Plugin.h

@@ -0,0 +1,17 @@
+
+#pragma once
+
+#include <zeek/plugin/Plugin.h>
+
+namespace btest::plugin::Testing_Plugin2 {
+
+class Plugin : public zeek::plugin::Plugin
+{
+protected:
+	// Overridden from zeek::plugin::Plugin.
+	zeek::plugin::Configuration Configure() override;
+};
+
+extern Plugin plugin;
+
+}