patch 8.2.0724: Vim9: appending to buffer/window/tab variable not tested

Problem:    Vim9: appending to buffer/window/tab variable not tested
Solution:   Add a test.
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 7768eb0..564e51d 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -135,18 +135,24 @@
       assert_equal('yes', b:existing)
       b:existing = 'no'
       assert_equal('no', b:existing)
+      b:existing ..= 'NO'
+      assert_equal('noNO', b:existing)
 
       w:newvar = 'new'
       assert_equal('new', w:newvar)
       assert_equal('yes', w:existing)
       w:existing = 'no'
       assert_equal('no', w:existing)
+      w:existing ..= 'NO'
+      assert_equal('noNO', w:existing)
 
       t:newvar = 'new'
       assert_equal('new', t:newvar)
       assert_equal('yes', t:existing)
       t:existing = 'no'
       assert_equal('no', t:existing)
+      t:existing ..= 'NO'
+      assert_equal('noNO', t:existing)
     enddef
     call Test_assignment_local_internal()
   END
diff --git a/src/version.c b/src/version.c
index 3084d9b..38da744 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    724,
+/**/
     723,
 /**/
     722,