patch 8.1.0723: cannot easily run specific test when in src/testdir

Problem:    Cannot run specific test when in src/testdir the same was as in
            the src directory.
Solution:   Move build rule to src/testdir/Makefile.
diff --git a/src/version.c b/src/version.c
index 1c08ebe..aede1e6 100644
--- a/src/version.c
+++ b/src/version.c
@@ -796,6 +796,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    723,
+/**/
     722,
 /**/
     721,