patch 8.2.1791: Vim9: debugger test fails

Problem:    Vim9: debugger test fails.
Solution:   Use "var" instead of "let".
diff --git a/src/testdir/test_debugger.vim b/src/testdir/test_debugger.vim
index b6231cf..3b02241 100644
--- a/src/testdir/test_debugger.vim
+++ b/src/testdir/test_debugger.vim
@@ -859,7 +859,7 @@
     vim9script
 
     def DoAThing(): number
-      let a = 100 * 2
+      var a = 100 * 2
       a += 3
       return a
     enddef