patch 8.1.0592: the libvterm tests are not run as part of Vim tests

Problem:    The libvterm tests are not run as part of Vim tests.
Solution:   Add testing libvterm.
diff --git a/src/Makefile b/src/Makefile
index aca1236..0012309 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2107,7 +2107,7 @@
 # TESTING
 #
 # Execute the test scripts and the unittests.
-test check: scripttests unittests
+test check: scripttests unittests test_libvterm
 
 # Execute the test scripts.  Run these after compiling Vim, before installing.
 # This doesn't depend on $(VIMTARGET), because that won't work when configure
@@ -2159,6 +2159,10 @@
 run_message_test: $(MESSAGE_TEST_TARGET)
 	$(VALGRIND) ./$(MESSAGE_TEST_TARGET) || exit 1; echo $* passed;
 
+# Run the libvterm tests.
+test_libvterm:
+	cd libvterm; $(MAKE) -f Makefile test CC="$(CC)"
+
 # Run individual OLD style test.
 # These do not depend on the executable, compile it when needed.
 test1 \