Add a makefile to build and run the tests for the bsnmp library.

This is not automatically built or run but must explicitly be built
with 'make' and run with 'make run'.

tools/test/bsnmp/Makefile

@@ -0,0 +1,19 @@
+# $FreeBSD$
+
+CONTRIB=${SRCTOP}/contrib/bsnmp
+.PATH: ${CONTRIB}/tests
+
+PROG_CXX=ctest
+SRCS= main.cc asn1.cc snmp_parse_server.cc
+CFLAGS += -I/${CONTRIB}/lib
+CXXFLAGS+= -std=c++2a
+LIBADD= bsnmp
+
+CFLAGS += -DBOGUS_CVE_2019_5610_FIX
+MK_MAN= no
+
+.include <bsd.prog.mk>
+
+
+run:
+	${.OBJDIR}/ctest