create system test with asynchronous plugin

the test-async plugin uses ns_query_hookasync() at the
NS_QUERY_DONE_SEND hook point to call an asynchronous function.
the only effect is to change the query response code to "NOTIMP",
so we can confirm that the hook ran and resumed correctly.

bin/tests/system/hooks/driver/Makefile.am

@@ -0,0 +1,13 @@
+include $(top_srcdir)/Makefile.top
+
+AM_CPPFLAGS +=			\
+	$(LIBISC_CFLAGS)	\
+	$(LIBDNS_CFLAGS)	\
+	$(LIBNS_CFLAGS)		\
+	$(LIBISCCFG_CFLAGS)
+
+hooks_LTLIBRARIES = test-async.la
+hooksdir = $(abs_builddir)
+
+test_async_la_SOURCES = test-async.c
+test_async_la_LDFLAGS = -avoid-version -module -shared -export-dynamic