patch 8.1.1136: decoding of mouse click escape sequence is not tested

Problem:    Decoding of mouse click escape sequence is not tested.
Solution:   Add a test for xterm and SGR using low-level input.  Make
            low-level input execution with feedkeys() work.
diff --git a/src/evalfunc.c b/src/evalfunc.c
index b79284c..7f8330f 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -3792,7 +3792,7 @@
 
 		if (!dangerous)
 		    ++ex_normal_busy;
-		exec_normal(TRUE, FALSE, TRUE);
+		exec_normal(TRUE, lowlevel, TRUE);
 		if (!dangerous)
 		    --ex_normal_busy;