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/auto/configure b/src/auto/configure
index 1c00773..61b7272 100755
--- a/src/auto/configure
+++ b/src/auto/configure
@@ -656,6 +656,7 @@
 X_CFLAGS
 XMKMF
 xmkmfpath
+TERM_TEST
 TERM_OBJ
 TERM_SRC
 CHANNEL_OBJ
@@ -7911,6 +7912,8 @@
 
   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"
 
+  TERM_TEST="test_libvterm"
+
 fi
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking --enable-autoservername argument" >&5