patch 8.1.0760: no proper test for using 'termencoding'

Problem:    No proper test for using 'termencoding'.
Solution:   Add a screendump test.  Fix using double width characters in a
            screendump.
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 6ea0c45..8ed4642 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -243,6 +243,7 @@
 	test_tagjump \
 	test_taglist \
 	test_tcl \
+	test_termencoding \
 	test_terminal \
 	test_terminal_fail \
 	test_textformat \
@@ -389,6 +390,7 @@
 	test_system.res \
 	test_tab.res \
 	test_tcl.res \
+	test_termencoding.res \
 	test_terminal.res \
 	test_terminal_fail.res \
 	test_textformat.res \