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/Makefile b/src/Makefile
index c084463..90390e0 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2193,7 +2193,7 @@
 #
 # Execute the test scripts and the unittests.
 # Do the scripttests first, so that the summary shows last.
-test check: unittests test_libvterm scripttests
+test check: unittests $(TERM_TEST) scripttests
 
 # Execute the test scripts.  Run these after compiling Vim, before installing.
 # This doesn't depend on $(VIMTARGET), because that won't work when configure