patch 8.0.1048: no test for what 8.0.1020 fixes

Problem:    No test for what 8.0.1020 fixes.
Solution:   Add test_feedinput().  Add a test. (Ozaki Kiichi, closes #2046)
diff --git a/runtime/doc/eval.txt b/runtime/doc/eval.txt
index 445a680..f9c6ae3 100644
--- a/runtime/doc/eval.txt
+++ b/runtime/doc/eval.txt
@@ -2410,6 +2410,7 @@
 test_alloc_fail({id}, {countdown}, {repeat})
 				none	make memory allocation fail
 test_autochdir()		none	enable 'autochdir' during startup
+test_feedinput()		none	add key sequence to input buffer
 test_garbagecollect_now()	none	free memory right now for testing
 test_ignore_error({expr})	none	ignore a specific error
 test_null_channel()		Channel	null value for testing
@@ -8196,6 +8197,11 @@
 		Set a flag to enable the effect of 'autochdir' before Vim
 		startup has finished.
 
+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.
+		This function works only when with |+unix| or GUI is running.
+
 test_garbagecollect_now()			 *test_garbagecollect_now()*
 		Like garbagecollect(), but executed right away.  This must
 		only be called directly to avoid any structure to exist