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/configure.ac b/src/configure.ac
index 3ad223b..b59914b 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -2108,6 +2108,8 @@
   AC_SUBST(TERM_SRC)
   TERM_OBJ="objects/vterm_encoding.o objects/vterm_keyboard.o objects/vterm_mouse.o objects/vterm_parser.o objects/vterm_pen.o objects/vterm_screen.o objects/vterm_state.o objects/vterm_unicode.o objects/vterm_vterm.o"
   AC_SUBST(TERM_OBJ)
+  TERM_TEST="test_libvterm"
+  AC_SUBST(TERM_TEST)
 fi
 
 AC_MSG_CHECKING(--enable-autoservername argument)