patch 8.2.5147: flaky test always fails on retry

Problem:    Flaky test always fails on retry.
Solution:   Delete the created function.
diff --git a/src/testdir/test_search.vim b/src/testdir/test_search.vim
index 794dd1d..5422b33 100644
--- a/src/testdir/test_search.vim
+++ b/src/testdir/test_search.vim
@@ -353,6 +353,7 @@
   let elapsed = reltime(starttime)->reltimefloat()
   call assert_inrange(0.09, 0.300, elapsed)
 
+  delfunc Waitabit
   bwipe!
 endfunc