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
diff --git a/runtime/doc/testing.txt b/runtime/doc/testing.txt
index c5fa2e8..7fb24e6 100644
--- a/runtime/doc/testing.txt
+++ b/runtime/doc/testing.txt
@@ -52,6 +52,11 @@
 		startup has finished.
 
 
+test_clear_search_pat()				*test_clear_search_pat()*
+		Clears the last used search pattern (|/|) and the substitute
+		pattern (|:s|). This is useful for testing conditions where
+		these patterns are not set previously.
+
 test_feedinput({string})				*test_feedinput()*
 		Characters in {string} are queued for processing as if they
 		were typed by the user. This uses a low level input buffer.
diff --git a/runtime/doc/usr_41.txt b/runtime/doc/usr_41.txt
index 1bed01f..316eebc 100644
--- a/runtime/doc/usr_41.txt
+++ b/runtime/doc/usr_41.txt
@@ -963,6 +963,7 @@
 	assert_report()		report a test failure
 	test_alloc_fail()	make memory allocation fail
 	test_autochdir()	enable 'autochdir' during startup
+	test_clear_search_pat() clears the last used search pattern
 	test_override()		test with Vim internal overrides
 	test_garbagecollect_now()   free memory right now
 	test_getvalue()		get value of an internal variable