patch 8.2.4297: Vim9: not all code covered by tests

Problem:    Vim9: not all code covered by tests.
Solution:   Add a couple more tests.
diff --git a/src/testdir/test_vim9_script.vim b/src/testdir/test_vim9_script.vim
index 767abd4..28dd78b 100644
--- a/src/testdir/test_vim9_script.vim
+++ b/src/testdir/test_vim9_script.vim
@@ -1580,6 +1580,14 @@
     burp
   endif
 
+  if 0
+    if 1
+      echo nothing
+    elseif 1
+      echo still nothing
+    endif
+  endif
+
   # expression with line breaks skipped
   if false
       ('aaa'