patch 8.2.3292: underscore in very magic pattern causes a hang

Problem:    Underscore in very magic pattern causes a hang.  Pattern with \V
            are case sensitive. (Yutao Yuan)
Solution:   Adjust condition for magicness and advance pointer. (Christian
            Brabandt, closes #8707, closes #8704, closes #8705)
diff --git a/src/testdir/test_search.vim b/src/testdir/test_search.vim
index 1b16207..dd47f4f 100644
--- a/src/testdir/test_search.vim
+++ b/src/testdir/test_search.vim
@@ -1962,6 +1962,17 @@
   call assert_equal(['abc', 'ABC', 'Abc', ''],
         \ getline(1, '$'))
 
+  call setline(1, input)
+  call cursor(1,1)
+  " \Vabc should match everything
+  %s/\Vabc//g
+  call assert_equal(['', '', '', ''], getline(1, '$'))
+
+  call setline(1, input + ['_abc'])
+  " _ matches normally
+  %s/\v_.*//g
+  call assert_equal(['abc', 'ABC', 'Abc', 'abC', ''], getline(1, '$'))
+
   set smartcase& ignorecase&
   bw!
 endfunc