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/getchar.c b/src/getchar.c
index 623440e..5b5b3ed 100644
--- a/src/getchar.c
+++ b/src/getchar.c
@@ -2059,7 +2059,7 @@
 		    c = inchar(typebuf.tb_buf, typebuf.tb_buflen - 1, 0L);
 		    /*
 		     * If inchar() returns TRUE (script file was active) or we
-		     * are inside a mapping, get out of insert mode.
+		     * are inside a mapping, get out of Insert mode.
 		     * Otherwise we behave like having gotten a CTRL-C.
 		     * As a result typing CTRL-C in insert mode will
 		     * really insert a CTRL-C.
@@ -2755,6 +2755,10 @@
 		     * cmdline window. */
 		    if (p_im && (State & INSERT))
 			c = Ctrl_L;
+#ifdef FEAT_TERMINAL
+		    else if (terminal_is_active())
+			c = K_CANCEL;
+#endif
 		    else if ((State & CMDLINE)
 #ifdef FEAT_CMDWIN
 			    || (cmdwin_type > 0 && tc == ESC)
@@ -2898,8 +2902,8 @@
 	    }	    /* for (;;) */
 	}	/* if (!character from stuffbuf) */
 
-			/* if advance is FALSE don't loop on NULs */
-    } while (c < 0 || (advance && c == NUL));
+	/* if advance is FALSE don't loop on NULs */
+    } while ((c < 0 && c != K_CANCEL) || (advance && c == NUL));
 
     /*
      * The "INSERT" message is taken care of here: