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/testdir/Make_all.mak b/src/testdir/Make_all.mak
index ad95b1b..b979c97 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -194,6 +194,7 @@
 	    test_system.res \
 	    test_tcl.res \
 	    test_terminal.res \
+	    test_terminal_fail.res \
 	    test_textobjects.res \
 	    test_undo.res \
 	    test_usercommands.res \