patch 8.2.4296: Vim9: not all code covered by tests

Problem:    Vim9: not all code covered by tests.
Solution:   Add a few more tests for corner cases.  Fix hang when single quote
            is missing.
diff --git a/src/testdir/test_vim9_expr.vim b/src/testdir/test_vim9_expr.vim
index cf4347c..87b8014 100644
--- a/src/testdir/test_vim9_expr.vim
+++ b/src/testdir/test_vim9_expr.vim
@@ -1833,6 +1833,7 @@
 
   v9.CheckDefAndScriptFailure(['var x = "abc'], 'E114:', 1)
   v9.CheckDefAndScriptFailure(["var x = 'abc"], 'E115:', 1)
+  v9.CheckDefFailure(["if 0", "echo 'xx", "endif"], 'E115', 2)
 enddef
 
 def Test_expr8_vimvar()