patch 8.2.0212: missing search/substitute pattern hardly tested

Problem:    Missing search/substitute pattern hardly tested.
Solution:   Add test_clear_search_pat() and tests. (Yegappan Lakshmanan,
            closes #5579)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 1d282d2..26e7d12 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2848,6 +2848,7 @@
 test_alloc_fail({id}, {countdown}, {repeat})
 				none	make memory allocation fail
 test_autochdir()		none	enable 'autochdir' during startup
+test_clear_search_pat()		none	clears the last used search pattern
 test_feedinput({string})	none	add key sequence to input buffer
 test_garbagecollect_now()	none	free memory right now for testing
 test_garbagecollect_soon()	none	free memory soon for testing