patch 8.0.1083: leaking memory in input part of channel

Problem:    Leaking memory in input part of channel.
Solution:   Clear the input part of channel.  Free the entry. Move failing
            command test to a separate file to avoid bogus leak reports
            clouding tests that should not leak.
diff --git a/src/Makefile b/src/Makefile
index 27615fa..0cb0058 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2265,6 +2265,7 @@
 	test_taglist \
 	test_tcl \
 	test_terminal \
+	test_terminal_fail \
 	test_textobjects \
 	test_timers \
 	test_true_false \