patch 8.0.1786: no test for 'termwinkey'

Problem:    No test for 'termwinkey'.
Solution:   Add a test.  Make feedkeys() handle terminal_loop() returning
            before characters are consumed.
diff --git a/src/keymap.h b/src/keymap.h
index 8a34f0e..d6dd5cb 100644
--- a/src/keymap.h
+++ b/src/keymap.h
@@ -270,6 +270,7 @@
     , KE_FOCUSGAINED = 98	/* focus gained */
     , KE_FOCUSLOST = 99		/* focus lost */
     , KE_MOUSEMOVE = 100	/* mouse moved with no button down */
+    , KE_CANCEL = 101		/* return from vgetc() */
 };
 
 /*
@@ -455,6 +456,7 @@
 
 #define K_IGNORE	TERMCAP2KEY(KS_EXTRA, KE_IGNORE)
 #define K_NOP		TERMCAP2KEY(KS_EXTRA, KE_NOP)
+#define K_CANCEL	TERMCAP2KEY(KS_EXTRA, KE_CANCEL)
 
 #define K_MOUSEDOWN	TERMCAP2KEY(KS_EXTRA, KE_MOUSEDOWN)
 #define K_MOUSEUP	TERMCAP2KEY(KS_EXTRA, KE_MOUSEUP)