patch 8.1.0597: cannot run test_libvterm from the top directory

Problem:    Cannot run test_libvterm from the top directory.
Solution:   Add test target in toplevel Makefile.
diff --git a/src/version.c b/src/version.c
index 577be31..a85295a 100644
--- a/src/version.c
+++ b/src/version.c
@@ -800,6 +800,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    597,
+/**/
     596,
 /**/
     595,