patch 8.0.0804: terminal window functions not yet implemented

Problem:    Terminal window functions not yet implemented.
Solution:   Implement several functions. Add a first test. (Yasuhiro
            Matsumoto, closes #1871)
diff --git a/src/Makefile b/src/Makefile
index e522c78..e4ccf37 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2256,6 +2256,7 @@
 	test_tagjump \
 	test_taglist \
 	test_tcl \
+	test_terminal \
 	test_textobjects \
 	test_timers \
 	test_true_false \