Filter improvement: LP and HP filters, added test function

The -3 dB level at cut-off frequency is now maintained for multiple
iterations.

src/test/Makefile.am

@@ -2,11 +2,21 @@ AUTOMAKE_OPTIONS = subdir-objects
 AM_CPPFLAGS = -Wall -g $(all_includes)
 
 noinst_PROGRAMS = \
+	test_filter \
 	test_compandor \
 	test_emphasis \
 	test_dms \
 	test_sms
 
+test_filter_SOURCES = test_filter.c dummy.c
+
+test_filter_LDADD = \
+	$(COMMON_LA) \
+	$(top_builddir)/src/common/libcommon.a \
+	$(ALSA_LIBS) \
+	$(UHD_LIBS) \
+	-lm
+
 test_compandor_SOURCES = test_compandor.c
 
 test_compandor_LDADD = \