patch 8.1.2203: running libvterm tests without the +terminal feature

Problem:    Running libvterm tests without the +terminal feature.
Solution:   Only add the libvterm test target when building libvterm.
diff --git a/src/config.mk.in b/src/config.mk.in
index e20a10f..6aa8429 100644
--- a/src/config.mk.in
+++ b/src/config.mk.in
@@ -87,6 +87,7 @@
 CHANNEL_OBJ	= @CHANNEL_OBJ@
 TERM_SRC	= @TERM_SRC@
 TERM_OBJ	= @TERM_OBJ@
+TERM_TEST	= @TERM_TEST@
 
 RUBY		= @vi_cv_path_ruby@
 RUBY_SRC	= @RUBY_SRC@