patch 9.1.1506: tests: missing cleanup in Test_search_cmdline_incsearch_highlight()

Problem:  tests: missing cleanup test_override('char_avail', 0) in
          Test_search_cmdline_incsearch_highlight().
Solution: Add the missing cleanup (zeertzjq).

closes: #17655

Signed-off-by: zeertzjq <zeertzjq@outlook.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/testdir/test_search.vim b/src/testdir/test_search.vim
index 8ec513e..446d5ec 100644
--- a/src/testdir/test_search.vim
+++ b/src/testdir/test_search.vim
@@ -843,6 +843,7 @@
 
   " clean up
   set noincsearch nohlsearch
+  call test_override("char_avail", 0)
   bw!
 endfunc