patch 8.1.2310: no proper test for directory changes in quickfix

Problem:    No proper test for directory changes in quickfix.
Solution:   Add a test that uses multiple directories. (Yegappan Lakshmanan,
            closes #5230)
diff --git a/src/version.c b/src/version.c
index 5030d7b..35dc4e2 100644
--- a/src/version.c
+++ b/src/version.c
@@ -742,6 +742,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    2310,
+/**/
     2309,
 /**/
     2308,